US T-Mobile HTC One not working in Germany

I have a US T-Mobile branded HTC One. It works fine in the US and we enjoy LTE speeds now in Minneapolis.

I received the official SIM Unlock code from T-Mobile.

Then travelled to Germany to buy a local SIM card.

I put the new SIM in but the phone never asked to input the SIM unlock code.

Everything works but the data (phone and SMS works fine).

Restarted many times, with SIM drawer out, in, etc.. same 'spiel' - can't get data to work at all.

The SIM is from 'Congstar', a low cost version of Germany's T-Mobile, but supposedly using their network and bands.

Do I need to put in a different, more 'foreign' SIM card? Whatelse could it be? Anything else I am doing wrong?