The Deco is in whatever mode it starts in: i believe Router mode. I am fine with having different subnets in the 2 buildings.

@Ryan yes, the WAP has a WAN port and LAN ports. And my mistake: the thing I called a router is a cable modem with 2 LAN ports. So actually, I want to do this, connecting 2 buildings on the site:...

Forum won't let me update, but to clarify, the WAP in the middle of the first diagram is connected to the switch by its WAN port and to the Deco by its LAN port.

I set up a Deco S4 successfully by plugging its ethernet cable into my router. I also successfully set it up after a reset, with this topology: Router -- tp-link switch -- WiFi access point -- Deco...