@Dilligaf74 

First, as others have said, you have to ensure your gateway computer (acting as router for all Ethernet packets, DHCP server, firewall) or some other device handles all Ethernet routing (of wired packets) for your entire network (including packets to/from both AP extenders). That is because, as others said, the RE615X can't do the required packet routing while it's configured as an Access Point. A traditional Wi-Fi router/Ethernet router does this. APs don't do this. So now, you need to provide this capability for your entire wired LAN, since you're removing your Turris router (it did the LAN routing).

After you've done the above, you can define the same pair of SSIDs for both RE615X APs (one SSID & password for 5 GHz on both APs, another SSID & password for 2.4 GHz on both) so that any roaming devices within your home will always connect to one or the other AP. Technically, this will not setup the APs as a "mesh" network but the above setup will give you some benefits of Mesh networks without having them actually operating as a mesh. (your informal use of the term "mesh" may have implied to others that you want/need a formal mesh network. To me it sounds like this is not one of your requirements, but you simply want devices to connect to one AP or the other, whichever is closest). This setup should accomplish what you want, but it's essential that packet-switch/router capability is somewhere on your Ethernet LAN (and both AP's are plugged-in/wired into this integrated LAN). More than likely, your gateway computer should do this, if it's configured properly.

If you encounter problems, most likely the gateway computer's configuration should be checked, to make sure the Access Points are being managed/integrated correctly (for example, maybe your LAN has more than one subnet: that overlap, are mis-configured, or your firewall won't allow traffic to pass between subnets).

Caveats/issues you should understand: this setup does not provide real "Mesh" networking features. For example, in your setup (as you defined), any roaming device that is in an area of your home where it gets signal from both APs will have difficulty staying connected to the same AP (since both share the same SSID, and since both signals are strong enough to connect to). In such areas of signal overlap, your device (e.g. a phone) will randomly swap between the APs and each time it swaps, your device may experience a few seconds (usually just a second, but up to 5 seconds while it establishes a re-connection). This random swapping between both APs is unavoidable, and is one of the reasons why true "Mesh" networks were developed. If your device/phone is buffering incoming data (e.g. while streaming audio/video), then you probably won't be affected by the swapping. But if your device is uploading (to the net), this will cause slight delays each time your phone swaps between APs. Also, if you don't have devices that roam within your house, this swapping will still occur for devices that are in a very narrow part of the overlapping area, but it should be less. It's difficult to estimate the effect of many local variables (like your house's shape, how its structure/furniture/walls reduce your signal strength in some areas, how you move within your home, etc.), so you have to try the configuration to know how well it will work in your home & local Wi-Fi environment. You may need to move one or both APs a few feet, or move one antenna on one/both RE615X, to optimize the Wi-Fi signals.

A note on antenna placement: Since each RE615X has two movable antennae, you can and should position them to maximize signal area (and minimize interference from objects in your home). Signals propagate perpendicular to an antenna, so position each antenna so that it "points orthogonally" to the areas you want to maximize signal (to your home's upper/lower floors). That is, imagine a giant flat sheet (a mathematical plane) sitting at the top of an antenna, and "aim that flat sheet" to the areas you want to have coverage. Do the same for the other antenna, to target a 2nd area for coverage. Make sure one antenna on each AP "orthogonally points to" the other AP (so they have strong signal between them). This last point is critical, for your setup to work reliably.

Hope this helps, and the setup works for you.

  @Ryan and @terziyski 

Thans very much for the reply.  Yes, I already have the RE615X units.  However, I was able to get things set up in such a way that the two do use my gateway coomputer for all of the necessary functions, and as @HelpingOne 

Bob

  @HelpingOne - thank you very much for your detailed reply, and I do have it set up exactly as you described.  The gateway computer performs all of the necessary functions - firewall, DHCP server, routing (I run my own DNS servers as well, and have 1.1.1.1 as a fall-back), etc. - and the two extenders use the same two SSIDs, one for 5 GHz and one for 2.4.  Yes, there is a slight lag when moving from one to the other, but the speed upgrade is worth it - up to 10x faster download speeds compared with my old router, probably will be even better when I have fiber access installed in a couple of months (waiting for the infrastructure to be finished around the city).

Also appreciated was the discussion of how the antennas are polarized.  Right now, I have one of the units handling the first floor and the other the second; both have the antennae pointed straight up, and it's working fine so far.

Thanks again for the very helpful reply.

Bob