4g can connect but 3g can not

Hello guys,

i am using this phone for 2years without any problem.
But 2months ago my second one is started not connecting to 3g network.

This causes me blocking calls and making calls.
ex: when connected to 4g I am trying to make a call and the phone is trying to drop connection to 3g network so it can not connect to 3g and call drops.

Also it is same when someone calling me. When someone calls me the gsm signal arrives to phone, phone tries to reduce connection to 3g, 3g can not connect, and the phone becomes unreachable.

What can cause this problem ?
Is this related with board ?
Firmware ? (tried flashing firmware so many times.)

Looks like your connection to Meizufans was lost, please wait while we try to reconnect.