mobile

I recently switched from T-mobile to o2. Of course there was a quality drop to be expected but now the phone seems to have problems to connect to the network after not having a signal. And even if it shows E, which is OK for where I live because the coverage is ****, I don't receive any data. The phone is not connected with the mobile network even though showing the E.

I thought maybe there is a modem, which works better with o2.