@brycehamrick If the operation mode of the Deco mesh is "Router", one Ethernet port of the main Deco must be exclusively used to connect to the modem.  The satellite Deco cannot communicate with the main Deco via that port.

 If the operation mode of the Deco mesh is "Router", you should probably disconnect the switch from the modem, and connect it to a free port of the main Deco.

  @yves_b thank you this makes sense. The location of the modem (which is also a router) and switch aren't in an area I want a deco in so I'll try access point mode.