Hardwired Wireless Access Point

Hey y'all, in a new townhome and my gateway is in the garage. I got a Zyxel wireless access point hardwired (with a POE injector) on the third floor. It is broadcasting, but when I have it broadcast as the same SSID as the modem it's only connecting to the modem, not the WAP despite it having the stronger singal upstairs. I assume now that I need to learn a bit more about making this a sort of mesh network but just a little confused.

I'm reading that I need to set the Modem to bridge, but if I do that do I then lose the ability to connect to the modem's wifi? The garage is next to a rec room so obviously the signal is better there with the gateway.

I'm just struggling with information overload because I feel like only having one WAP is a bit different than several repeaters which seems like it may be what I keep getting linked to.