I recently created a video of how to get straight line missions again. Ok, a bit of „hand work“, but when automating the task e.g. by creating an apple script quite easy to accomplish:
same for me - just sold my Mavic Air 2 and inserted the Sandisk Extreme Pro from there in the Air 3S as kind of backup: if I run out of space with the internal storage (what never happened so far, as I always download videos/pics after the flight).
From DJI with a nice diagram explaining how it works:
https://enterprise-insights.dji.com/blog/introducing-4g-connection-for-drones-with-dji-cellular-dongle
Just found this on the support website from DJI:
"The enhanced transmission requires a relay server to facilitate low-latency 4G video transmission coupled forwarding, the subscription fee covers the cost of renting servers and cloud services."...