Hi @Rat_Fink This is my first reply on this platform. Sorry if i will make any mistake.
5G Modem
→ There are enough 5G module available on the internet one that i can pick for you https://www.waveshare.com/usb-to-m.2-b-key.htm this one .(Please See Band availability and
operator support for this modem) and enough from quectel as well.
To Control Over 5G (I am assuming you already have companion computer for your rover.)
→ You can create your cloud based app for your drone/rover and control it over 5G (Example video link : https://www.youtube.com/watch?v=IokyotAGbJI ). In this video t is explained for quadcopter but similiar connection process for any vehicle.
→ arducloud is one of the options
→ and one more app that i have created during college days (https://idronam.com/) this is typically for quadcopters runnning ardupilot but you can send them mail to extend the support for rovers as well. Its free for individual/developers/hobbyist.
I hope this helps .
if there is any thing else please let me know.