@Jeffg53 

It is recommended not to place two WiFi broadcasting routers next to each other. This is general recommendation, not just for Deco. In your case, your two routers were Main Deco and Archer. 

There are two options to fix that issue: move Main Deco away from Archer, or turn off WiFi on Archer. You bought X20s to provide WiFi coverage at your place, it would make sense to turn off WiFi on Archer. 

  @Alexandre. Many thanks. I assumed that that was probably the issue. I will turn off wifi on my modem and put the other Deco next to it again. 
 

I have been working on networks and comms since 1971 but only cable based. I'm grateful for your assistance. 

  @Jeffg53 

As you are going to keep Archer router, I have one more recommendation: configure Deco mesh to run in Access Point mode if you haven't done that yet. In AP mode Deco will provide WiFi coverage, while you will keep managing network through Archer router. 

That will help to avoid double NAT issues and will maximize Deco mesh WiFi performance.

See How to set up Deco to work in Access Point mode

  @Alexandre. Many thanks. I'm away at the moment but I'll do that as soon as I get back.

  @Jeffg53 I'm back and installed the new modem/ router, and turned off wifi on it. I now have only Deco for wifi. When I was using it on router mode, all was good. I then changed to access point mode and things started to go wrong. The most obvious was that my inbound speed was all over the place. I left it for a while, changed back to router, and all is good again. 
 

Any suggestions, I would like to get going in AP mode if I can. 

  @Jeffg53 and my latest discovery is that my Garmin devices can't connect to the Deco. I hope that this is another known issue

Jeffg53 wrote

  @Jeffg53 I'm back and installed the new modem/ router, and turned off wifi on it. I now have only Deco for wifi. When I was using it on router mode, all was good. I then changed to access point mode and things started to go wrong. The most obvious was that my inbound speed was all over the place. I left it for a while, changed back to router, and all is good again. 
 

Any suggestions, I would like to get going in AP mode if I can. 

  @Jeffg53 

Usually it is other way around: in AP mode Deco just broadcasts WiFi and passes traffic. It can't be worse than in router mode. Something else might have been interfering with your Deco mesh WiFi signal while you were running it in AP mode. Try again and give it more time.

  @Jeffg53 I will try it again. It went wrong straight away but I will leave it and report back. Will you be looking at the Garmin question that I have raised?

  @Jeffg53 

OK. I tried it again and left it for a couple of hours. As soon as it is in AP mode, my inbound broadband speed starts jumping around. The Deco units start dropping the connection to the main unit, and the main unit keeps on dropping out.

As soon as I set it back to router mode, everything is good again.

I'm at the end of my tether. I bought Deco because I like TPLink gear. All I can think is that the units that I have are duds.

I'm ready to give up. I just don't have the time or patience for this.

  @Jeffg53 

I just rang TPLink support. Their advice is that AP mode will not work with my modem and that I should stay on router mode. They also advised me to set my wifi to 2.4 only, connect the Garmin devices, and then set it back to 2.4 and 5.  So far so good