@Renard 

I may be able to help with improvements to your home network and WiFi, but this may not be the fast process. I will need to ask questions, follow-up questions, to get information from your that will define recommendations. I'll try to help with WiFi performance improvements and see if I can recommend better separation between network users in the house and guests. This conversation may take days, it won't be fast - we do it online, not in person.

If you could find someone with relevant technical skills who could set everything for you on premises, that might be better.

Having said that, if you want to see what help you can get on this forum, lets start with me collecting more information from you:

1. ISP router is located in guest accomodation, and you have house hardwired. Based on number of ports in the house, you must have networking switch somewhere between the house and ISP router.

Where is this switch placed (house, guest accomodation)? What is switch brand and model number?

2. ISP routers usually have their own WiFi. Why haven't you just enabled router's WiFi for guests?

3. Do guests have physical access to ISP router, or is it in the locked cabinet? 

4a. Do you need same WiFi network (same WiFi mesh) in both guest accomodations and the house?

4b. Do you need non-interrupted WiFi connectivity between these two building, meaning when you walk from the house to guest accomodations and back WiFi signal absolutely must not drop even for a moment in between these two places.

5a. Have you decided on your own to purchase three-unit set of Deco E4, or someone recommended that for you?

5b. Who configured Deco mesh: you, someone else?

6. Just to be clear, "cottage" and "guest accomodations" is the same place, or are these two different places?

  @Alexandre. 

Hi, I'll start with the easy stuff first.

6. The guest accommodation and cottage is the same building.

5b. I configured the mesh myself. 

5a. Have researched online decided that the Deco E4 units were the best option for us, we have very thick walls, 800mm thick, there are 2 external walls between the cottage and our house which are 20 meters apart. So, we needed the thick wall, large house capability as well as the range of coverage the 3 units gives. 

4b. We have that at the moment but it is not essential.  

4a. I don't know, at present 1 Deco unit is beside the router in the cottage at the moment as the master and the other 2 are in our house. 1 downstairs, 1 upstairs.

3. The router is in open view. This is due to where the fiber cable enters the cottage. 

2. From the router we have 2 accounts set up, one for us and another for guests, separate passwords. 

1. The networking switch is in the house, it is a GIGAMEDIA GGM GS08D, 6 ports are connected in the house so 2 are free. 

If the explanation is kept simple without the use of technical terms/names I can usually manage to install things myself. In this situation I just don't know what to get or how to go about doing what I am trying to achieve, if it is even achievable. 

Thanks in advance.          

  @Renard 

In our conversation my first objective is to create network diagram for your place: what is connected to where. Then, I will need to figure how Deco mesh fits in this picture, I still have questions to ask.

After I have that, I can provide recommendations, based on what you have. They will focus on improving WiFi performance for guests and on protecting your house WiFi and wired network.

But first, a bit of bad news: Deco E4 has 100Mbps Ethernet ports, not gigabit. Anything that connects to Deco E4 will not get speeds faster than 100Mbps. This Deco model is not a good fit for Internet connections faster than 100Mbps. You said you are getting almost 700Mbps from fiber.

We can return to the topic of proper Deco model later, but I need you to clarify for me: do your guests get WiFi from ISP router or from Main Deco? Simply speaking, if you power off Main Deco E4 in the cottage, will there still be WiFi coverage?

More questions:

1. Do you have Ethernet cable between the house and cottage, connecting switch in the house to ISP router in the cottage?

2. In the house, do Satellite Deco E4 connect to Ethernet ports with Ethernet cable, or do they connect wirelessly to Main Deco in the cottage?

  @Alexandre. 

If the main Deco in the cottage is powered off guests would still get WiFi

1. Yes, there is a Ethernet cable connected to the ISP router in the cottage which is connected to the switch in the house.

2. The 2 Deco E4 in the house connect wirelessly to the main Deco in the cottage. 

Renard wrote

Would it be better to disconnect the main deco and just have it connected wirelessly, having checked the packaging they can operate at 300 Mbps on 2.4GHz and 867 Mbps on 5 GHz

  @Renard 

Unfortunately, Main Deco must be connected by Ethernet cable. Don't worry at this point, we may be able to use that for troubleshooting, later.

I think I have everything to start with recommendations. One (hopefully, last) question: your Deco E4 set, can it still be returned to retailer and if yes - when will return window end?

I like to work with networking gear that is already in place, but if you can return Deco E4 I will consider that as an option in my recommendations.

  @Renard 

I am going to start with an assessment of what you have, and then will provide you with the plan that you should be able to execute now, and without need for getting new or additional equipment.

You have ISP router in cottage, where guest are. They have physical access to it. In theory, it means they can connect to the router not only by WiFi, but with Ethernet cable to its ports. They can even reset it and bring back to original manufacturer settings. Whatever security configuration settings are made on that router are not guaranteed to stay.

You are correctly concerned with security of your house network, which is connected by Ethernet cable to ISP router in the cottage. As it is now, someone may be able to get access to house network from the cottage. 

We are not going to secure ISP router. Instead, we are going to consider it to be part of the Internet, where everything goes. The Ethernet cable that comes to your switch in the house will be treated as Internet link.

The standard way of protecting home network from the Internet is firewall. It comes by default with any WiFi router you could buy, and also on Main Deco. We are going to use Main Deco with its firewall to protect house network.

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You also mentioned that Internet could get sluggish when there is a lot of guests activity. If guests do complain about it, this could be for multiple reasons. It could be because you have both ISP Router and Main Deco close to each other: both broadcast strong WiFi signal which could create interference. This could be because someone or something in house network downloads a lot, adding to congestion from guests. Finally, it could be just because you need faster Internet.

I'll help you remove two out of three (except faster Internet), and you'll see if guests stop complaining. If not, you'll have to sign for faster Internet.

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This completes my assessment. The next post with be the plan I'll suggest for you.

  @Alexandre. 

Thank you. 

Prior to trying this I checked the wifi speed in the downstairs of the cottage and was getting a speed of 19Mbps. I unplugged the master Deco from the router and did another speed test from the same spot as the first, I received a speed of 121Mbps. 

The master Deco was then plugged into the switch in the house and I moved the Deco which was in that room to another room and conducted speed tests throughout the house, where before we were getting around the mid 30Mbps throughout the house we are now getting 98Mbps. 

Thanks for your help