Redmi 1S lose 3G connection topic

Some months ago my phone started to behave strangely.

I have 2 sim card both for Vodafone. I use the data connection from the 1. sim.
Once i start a call from the second SIM card the phone drop the data-connection from the first sim(as expected) but after the call end the phone seemingly connect back to the 3g network but I cannot gat a byte from the internet.
I tried to switch between the cards and the result was that if I use the 2g in the second slot and start a call from the first sim the phone easily connect back to the internet after start a call.
also this is the case if the data connection in the 1. sim is 2g and not 3g.


So the main problem as i can see is that if i start a call from the second sim when i use 3g data-connection on the first sim after the call ends the phone cannot connect back properly.


If I turn on and off again the data-connection is not help.
But if i switch between 2g and 3g on the first sim the internet can be used again.

Please advice.

Shall I sand it back to the Chinese retailer? from Hungary:(