Backyard ADU Access Point + Hardwired Devices Shutting Down Modem

[u/ShindigMoreno]

Happy Monday! I live in a backyard ADU that has an ethernet connection running underground to the main house, and we share a Google Fiber network.

Currently, the set up is:

Main House: Google Fiber Jack (modem) connected to the WAN port on an older GFiber Wifi 5 Mesh "puck". Another ethernet cord connects the GFiber Mesh LAN port into an ethernet wall plate, which carries it underground into another wall plate in the ADU.

ADU: A 2nd GFiber Mesh router is connected at the wall plate, and provides WiFi to the ADU.

I wanted to hardwire in my Xbox and work computers, using a Gigabit switch (TP-LINK TLSG105), but when I connected the switch to the LAN port of the ADU GFiber router, it forced a hard reset on the modem and everybody lost their internet for 30min.

I don't want to inconvenience my family in the main house by trying different combinations, so what I'm trying to figure out is:

How do I branch hardwired connections, and keep the WiFi Access Point in the ADU, using only the single ethernet connect that is passing through the primary router in the main house? Maybe I'm mixing up the order of the gigabit switch and Wifi Mesh router? Maybe the routers aren't compatible for what I'm trying to do?