Xfinity XB8 and Asus RT-AXE 7800

@user_1stvmd Thank you for reaching out via our Xfinity Forums. We are not trained in 3rd party mesh networks. Please reach out to the manufacture for assistance. 

It would probably work, but wouldn't be "mesh."  Most WiFi routers that don't support a cable connection have an access point mode.  I just can't verify that this one does since Asus doesn't have a manual on the downloads page for it.  Asus just has a 2 page quick start guide on the download page.  Useless.  For that reason I recommend getting something else unless you can figure out where the manual is.  IMHO no manual = deal killer.

If you already have it, just look around in the config settings and see if you can put it in access point mode or disable DHCP.  You'll want wire a computer to the router to do the setup.  Using the WAN port to connect to the XB8 may not work in access point mode and probably won't work if you're just disabling DHCP.  So if you can't find any useful docs I'd try a LAN port first.

Assuming the RT-AXE 7800 has an access point mode (probably does but I won't vouch for it) you could wire it to your XB8 and give it the same SSID (WiFi network name) and password and devices would be able to connect to either of them.  You just won't get fast roaming or other "mesh" features.  Fast roaming gives hints to clients to switch between WiFi nodes, so you get better speed when moving around your home, office, etc.  If you don't have fast roaming I recommend putting your laptop to sleep when moving around.  It will find the best signal when you wake it up again.

On the consumer side "mesh" means a group of WiFi nodes that work together and support things like fast roaming and/or have the ability to communicate with each other over WiFi, but consumer "mesh" can still be "mesh" if you wire the nodes together.  At least in theory.  On the commercial side "mesh" is WiFi access points talking to each other over WiFi.  Basically the whole "mesh" thing for consumer grade equipment is the marketing department running amok and you really need to check the specs to see what you're actually getting.  Some "mesh" gear is just a router and a couple range extenders.  

You'll probably want to put the XB8 in bridge mode or buy a cable modem once you get a second Asus router or other Aimesh compatible product.

If you can do it without causing too much "ugly" (cables strung across a room, etc. -- too much ugly is personal preference) and the wired connection is fast enough it's generally preferable to wire WiFi nodes together rather than have them talk to each other over WiFi.  Wired connections are less susceptible to interference, don't "waste" wireless bandwidth that could be used for client devices talking to the router, and can allow more optimal placement of WiFi nodes.

I'll use my house as a placement example.  I live in a deep but narrow city house.  I want reasonably fast WiFi in the living room at the front of the house and in my office at the back of the house.  The middle (kitchen, dining room, and bedrooms) I don't care so much about as long as I have decent coverage.  If your WiFi nodes are talking to each other wirelessly the optimal placement is roughly halfway in between.  So if I put my router in my office I'd want to put the second one in the middle of the house for optimal speed in the living room.  With a wired connection I can put the second one in the living room where I want a good signal.  I have an unfinished basement so I just run cables across the ceiling in the basement and install jacks in the walls on the first floor.

Depending on your speed requirements one thing you might want to think about for your next purchase is getting a router with two 2.5Gb ethernet ports, one LAN and one WAN.  The RT-AXE 7800 only has a 2.5Gb WAN port.  All the LAN ports are 1Gb.  It's not important if you don't see needing more than 950mbps any time soon, so 800 speed plans or lower.  Comcast likes to overprovision a bit, so you can usually get 20% more than your plan speed, thus 800 is as high as you can go without running into the limits of 1Gb ethernet.  It's also not important if you're not going to wire any devices, don't mind if the remote WiFi node doesn't have full speed, or plan to just let the nodes talk to each other over WiFi.  If you want to wire a computer or get max throughput out of that second WiFi node you'll need two 2.5Gb ports on the router.  No need for more than one 2.5Gb LAN port though.  You can just get a 2.5Gb switch and go router-switch-2nd WiFi and connect any other wired 2.5Gb devices to the switch.