I have 2 x BR1 Pro 5G routers in my RV (I’ll call them Router1 and Router2). I do this primarily to have an additional cellular radio and to support 4 sim slots in my setup. I only need one ROUTER which is what Router1 does. It would be great if I could set Router2 as an Access point and let Router1 do all the DHCP work and use the WAN sources in Router2 to pass to Router1 for Internet connectivity and use Router2’s Wi-Fi as an Access Point (same SSID) to work with Router1. Router1 uses paddle antennas for Wi-Fi for indoor connectivity, a roof antenna for cellular; Router2 has a 7-in-1 antenna on the roof for external Wi-Fi and cellular.
That is what I do. You basically need 2 patch cables going between the devices.
For router1 (192.168.50.1) using router2 (192.168.50.2) as WAN traffic I utilize a “dmz” VLAN which is different IP range from my local LAN. (ethernet wan on router1 (192.168.253.1) connects to dmz VLAN (192.168.253.2) on router2) - LAN1 port on router2 is setup to be in “dmz” VLAN. This keeps things tidy has several advantages.
On router2 - I setup the client SSID’s/AP to be on the default VLAN and then I have a patch cable between LAN2 on each device and that port on each device. This bridges default VLAN across devices.
On router2 I have a static route for my pepVPN-enabled private IP ranges pointing to 192.168.50.1. So router2 can be managed remotely without needing its own pepVPN tunnel. If you dont do this connectivity to router2 from other sites wouldnt be possible.