Been there. Most smart cams and baby monitors only join 2.4 GHz. If your router combines 2.4 and 5 under one name, create separate names or temporarily turn off 5 so the monitor cannot pick the wrong band. Put your phone on the same 2.4 network during setup and disable cellular data and VPN so the app does not wander off.
Check WiFi security. Many older models want WPA2 with AES only, not WPA3 or mixed WPA2 WPA3. Avoid WEP and avoid TKIP. Simple network name and password help as well. No spaces at the end and no special characters like ampersands.
If it still refuses and look for MAC filtering on the router and turn it off, then try adding the monitor again while standing close to the router. Once it connects, you can move it back to the nursery.
