I wish I had found this thread a week ago!
I had the same problems with fully greyed out data options, and went through lots of troubleshooting such as leaving the SU15 beta, port forwarding, etc as described in my thread PC - Stuck in offline mode, all data options greyed out
I finally isolated the issue to my ISP Talkmobile, which was surprising to me, so I googled “talkmobile microsoft flight simulator” and got here.
Findings from my testing are:
Standard broadband: works fine
4G via EE sim card: works fine
4G via Vodafone sim card: works fine
4G via Talkmobile sim card: does not work, as described in this thread and my thread
4G via Talkmobile sim card and VPN: works fine
What surprises me is that Talkmobile is owned by Vodafone and my 4g router is connecting to the exact same cell tower, frequency band, speed, signal, and even reports it as Vodafone - I can only guess Talkmobile are blocking with software on their side somewhere or as described by @BegottenPoet228 . Additionally Talkmobile is not known to have different access to Vodafone, as shown on this crowd-produced UK network comparison sheet UK Networks and MVNO spec - Google Sheets
It’s also surprising how all other internet usage (including successfully logging into Microsoft Store and XBOX app) is fine, it’s just this authentication on MSFS that is blocked.
I wonder if MS/Asobo could give any detail on what servers (or any other detail) are specifically required for the MSFS authentication and then I could pass this on to Talkmobile to see if they can remove the blockage on their side? I have tried manually setting DNS on PC and on the router itself to 8.8.8.8 but no change