Default lenovo a820 problems with wcdma/gsm slot

Hi all,

I bought a lenovo a820 which has a dual sim.
I also have an unlimited packet with 3g internet and mobile calls.

The GSM slot works perfect but i have just a 2g internet there .
The WCDMA/GSM slot does not work at all.
The phone identifies the 3G network but cannot connect to it.
I even cannot perform calls then.

I made a research on google and found that with lenovo p780 they had the same problem but in a different direction.
There the 3g slot did work but not the 2G slot

What may i do to solve the issue?