used to have spotty coverage underground with vodafone, when i switched to telekom, internet suddenly magically worked underground on the routes i used.

I believe someone published a map of the data coverage of different providers on the berlin ubahn, but probably outdated now

Yeah, admittedly this year I've also started experiencing holes on the ringbahn (strangely and consistently around frankfurter allee), but the ubahn has been fine.

I'm with sim.de which I believe is essentially an O2 reseller (apn references o2)

Gesundbrunnen too.

Wow! I was in Berlin last week and kept losing connection... like all the time. I use 3 with a Swedish plan. In Sweden, it literally never drops, not on trains, not on metro, not on faraway mountains... it works everywhere.