German LTE (4G) not available after using US SIM card…

I have a very weird problem… Since I have used a RedPocket prepaid SIM on a US trip, my iPhone 5 wont use LTE any more after I popped in my T-Mobile SIM here in Germany. No, LTE is not disabled in the settings. I have already tried turning it off and on again, disabled/enabled LTE and even reset my network settings, without resolving this issue. Instead of the "LTE" logo, I only get "3G" and the Speedtest app confirms I am only on 3G…

I think it's possible that the RedPocket profile has somehow messed up my network settings pretty good and/or t-Mobile is not overwriting the network settings properly…

Has anyone ever heard of this kind of problem?

Of course there might also be a problem with my T-Mobile account, for all I know they might just have turned of LTE for my account for whatever reason, but I haven't heard back from them yet…

Thanks and kind regards, Hans Schneider