Could you (or anyone else) try the following?
- Download BBS (Better Battery Stats free open source version) from F-Droid appstore
- Set it up and let it run until you hit the battery drain
- Open up the tab “Kernel Wakelogs” and check whether “RMNET-DFC” is high on the list.
- If it’s not that one, check which ones are high on the list
The reason for this is that I have the same issue on A12 (I’m waiting with updating until the freezes/crashes are fixed) and it’s caused by VoLTE.
My workaround is to disable VoLTE, which fixes the battery drain but now calls don’t get through if I don’t have 3G reception or the handover from 4G to 3G doesn’t work. Another option is to only use 3G.
I reported this to Fairphone, but they say, unless I upgrade to the newest version, they don’t want input from me.