Question about Yealink Sip-T48s

As Mr Heeb writes, you need a LAN connection at the location. Then it depends on how you telephone with Sunrise or whether you also have Internet access via Sunrise. You may have received a FritzBox from Sunrise. The FritzBox is a router and a small telephone system. You can order a second telephone number from Sunrise or from a VOIP provider (e.g. Sipcall). The access data is recorded in the FritzBox, i.e. the FritzBox establishes the connection to the telephone provider. The telephone itself then communicates with the FritzBox. If you have a Fritzbox from Sunrise, it is best to check with Sunrise first, because the devices are sometimes not fully activated.

If you have a VOIP-capable DECT base station, it can certainly manage several numbers. You can book the additional number with any provider. You must be able to connect a VOIP telephone to the router via LAN. Theoretically, you can also use a WLAN bridge. But it is not easy to set one up. I have already practised unsuccessfully, although I am not technically clueless.
In newer houses or with new electrical installations, a LAN connection via the mains is also possible. It's easy to install, but whether it works satisfactorily from a technical point of view is questionable.
A second DECT telephone on the same base is certainly easier.