Yes, you can use any data SIM in it to access the internet, the ones from Swisscom also work, of course. Or e.g. those from Digital Republic, but the download would not be higher than 300 Mbps.

It is simply important to place this type of device as close to a window as possible, but since the WLAN signal is then actually emitted in the wrong place, I would think about switching off the WLAN on this router and working with an alternative mesh WiFi solution, for example.

Of course, you could also use WLAN mesh hardware from Fritzbox, but these are usually more expensive than the solution from Xiaomi, which offers a very good WLAN mesh system with WiFi 6 with the AX3000 system. Maybe that would be something? The kit contains two WiFi mesh routers that can manage up to 256 devices, and could also be expanded with additional mesh routers if required. Each router also has an integrated 3 port Gbit switch.

Xiaomi AX3000