@ccrapp 

You want to keep your home network settings as they defined in your Asus AC68U, and only want to improve WiFi coverage. You can do that by running Deco mesh in Access Point mode. 

By default, after initial configuration, Deco mesh will run in Router mode. After you have configured Deco mesh and before you connected your home devices to it, switch Deco mesh to run in Acess Point mode, here is how: Switching a Deco to Access Point (AP) Mode

This is how I would recommend to add Deco mesh to your household:

1. Configure first Deco unit, follow directions from the user guide/manual/quick setup it came with. This will be your Main Deco. It must be connected to Asus AC68U with Ethernet cable. During configuration you will be asked to for SSID/password, make them unique and different from one in Asus AC68U.

2. After that, change Deco mesh to run in Access Point mode.

3. Add second Deco unit as Satellite Deco. How to add an additional unit to your Deco network

4. Verify that Deco mesh is set correctly: connect device to Deco SSID and see if it gets Internet access and has IP address in the 192.168.10.xxx range from Asus AC68U. If that device shows IP from the 192.168.68.xxx range, that would mean your Deco mesh still runs in default Router mode with its own default IP range. Figure why is that and fix that (try switching Deco mesh to AP mode again, do reboot of Deco units, etc.)

5. Having successfully completed all of the above, turn off Asus AC68U WiFi. All your wireless devices should disconnect at this point.

6. Configure Deco mesh with your Asus AC68U SSID/password combo. Most devices should reconnect to Deco mesh WiFi seamlessly and you'll see them in Deco app. Those that won't, you may need to troubleshoot them.

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You can connect PC to Ethernet port of Main or Satellite Deco and that PC will see it as a regular Ethernet connection. More to that, you can connect standard networking switch to free Ethernet port of Deco unit, and that will give you multiple Ethernet ports to wire devices to, if you need that.

Also note that in Access Point mode you can keep using all  Asus AC68U Ethernet ports for wired devices and they will be in the same network as everything that is connected to Deco mesh, wired and wireless.

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TP-Link offers huge variety of Deco mesh models. Among them, I would suggest you choose from those only: E4, M4 (also known as M4R) and M5. Each of these three comes with their own benefits and caveats, so if you need more info on each - I can share it with you, from the experience of Deco user.

Hi Alexandre, THX for that detailed writeup, much appreciated. I think I've understood how to get the mesh stuff up and running re the Wifi bits, seems straightforward.

Re the ethernet, each Deco E4 has two ports. So my understanding is that the "main" Deco (upstairs) would use one of those ports for the connection to the ASUS router and then the other is available for connecting a device, albeit at 100MBit only. OTOH I have three devices upstairs that currently are connected to the ASUS ethernet ports (at 1GBit) and I assume these wired devices can simply stay connected to the router.

The second Deco (downstairs) would have both ports free for connection. That's great as so far I have no ethernet downstairs... so I could connect up to two devices (which are currently on Wifi) to the Deco ethernet ports, again at 100MBit for the E4s. So... the question is whether I need GBit speeds downstairs. If yes, I would probably look at the M4 instead of the E4.

Any other major advantage of M4s vs E4s you can think of? Pricewise, here in the UK, the difference is not that dramatic, so I may well look at the M4s.

Again, thank you very much!

  @ccrapp 

If you are not on very tight budget, I would recommend to go with M4 instead of E4. You might not need gigabit Ethernet ports now, but you are buying Deco mesh to run it for few years. What if someone brings faster than VDSL Internet to your area in a year or two?

Also, if you have or plan to have NAS (file server) at your house, your home network will benefit a lot from having Deco with gigabit Ethernet ports when you move files to/from NAS with device such as PC, connected to Deco. 

There is no much difference between M4 and E4 otherwise. Both can do just fine with VDSL speeds and if you are confident you won't get faster Internet in 3-5 years and you don't need to move huge files through your home network - E4 could be good choice, too.

One thing you need to keep in mind: TP-Link does not sell E4 and M4 in single units. They come in 2-unit or 3-unit sets. So, you either need to decide upfront how many you'll need, or if you plan to add single one later - that is going to be different Deco.

M5's are sold in single units and they can run just fine with E4/M4. You can buy 2-unit set of E4 or M4 now and if you need one more Deco later - buy and add single M5. Of course, if you are very confident you'll need three Deco units - just buy 3-unit E4/M4 set now. Consistent hardware is always a plus when it comes to networking.

As for wiring devices to Deco units - it'll work the way you want it to. You can use free Ethernet port on Main Deco, both Ethernet ports on Satellite Deco. Your devices wired to Deco will report it as Ethernet link.

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One more recommendation. WiFi mesh (from any vendor) is hit an miss. It may work just fine, but you may find that some devices in you household can't handle WiFi mesh and won't connect reliably. This happens. So I would strongly recommend to get Deco set from the place with easy and seamless return policy. 30 days return window should be enough for getting to know your new Deco mesh system, figuring if there are any issues with your devices coexisting with it, and finding if these issues can be resolved.

The biggest challenge is usually older devices that can only connect on WiFi 2.4GHz. Such device might either not handle WiFi mesh (more than one node broadcasting same SSID) or will always stubborny connect to the node which is farther away, negating the point of WiFi mesh. 

If your devices support WiFi5 that usually a sign of less compatibility issues, otherwise buying WiFi mesh from the place with return policy is a must.

Again thanks, Alexandre! I will probably go for the GBit/M4 variant as the difference for twins is only around £15.

What you wrote about 2.4GHz devices is interesting. Among the zoo of Androids we have here there are a few older ones which support just 2.4, not 5. So either they'll work or they won't

They all currently work well with the AC68U, albeit somewhat slower downstairs. So if any of them won't work with the Decos I could probably just leave them on the AC68U 2.4G wifi. In which case I'll have to change the SSID of either the mesh or the router, I think.

Final question (famous last words): I do not have and don't plan to use a NAS as I've put a fast SSD on the AC68U's USB3 port and that works well enough for me. I assume that the router itself will be reachable under its 192.168.10.1 address from the whole network?

  @ccrapp 

I had similar setup to what you will: ISP router with external drive connected to USB3 port, and Deco mesh running in AP mode. Everything worked just fine: external drive and ISP router were accessible from any device on my home network, including those connected through Deco mesh.

I will report back once I have the whole thing up and running. And again thanks for your support, this was really helpful!