Why does the phone lose coverage?

Question

Android

There is a problem. Two weeks ago, in the morning, the mobile connection on the phone stopped working correctly. Phone Vowney 5, Android 4.2.1.
The problem is this. Phone for two cards. Both are determined, you can go to the SIM menu. And the coverage constantly disappears on one, then on the second map, then on both at once. On the street, the signal level is usually 40-60%, indoors and the bus disappears completely.
I untwisted it, the antenna to the GSM module is connected tightly, the 2g / 3g switch is on the first point. Didn't fall. Changed firmware - no result. What could be the problem?

Answer 1

The problem is most likely in the antenna, it happens often that the contact has moved away

Answer 2

And if you leave 1 sim card. all the same?

Answer 3

In this case, the problem is most likely in the manufacturer.