- Keep the DSL modem in bridged mode
- Archer C7 acting as a router, you can keep the WiFi enabled, but make sure it's on a different channel than the Unifi AP.
- Unifi AP (or ZyXEL has some good ones) in the middle of the apartment or at the other end if you decide to keep WiFi enabled on the C7. Just connect an ethernet cable from the router to the PoE injector and then another cable from the injector to the AP.
A network switch is an OSI layer 2 device, so it doesn't do any IP routing so you need the router to do that.