I have  BE900 and a GE 800 routers and am trying to set them up as GE800 in my bedroom as an access point with the BE900 as a mesh device at ythe other end of the home.  I need to do this as I have a fiber ont in my garage, which is under the home (which wifi barely gets thru) attached to a firewalla hardware firewall/router that also handles DHCP.  When I set the 800 as an access I lose the choice to ad a mesh devuce and if I mesh them and then select access point, I lose the 900 in the network map.

I had two older Asus routers that I could mesh as an access point and they worked well any suggestions as to how to set this up.