The first step is to check V+ Line whether the pin connections and the antenna is connected properly or not. If the pins are dirty or not well connected, do improvements in these areas.
The second step is to check the antenna path as shown below. Check one by one if there is a spur that is broken or not, if there is a path-breaking try doing jumpers carefully and thoroughly.
Copper wire (To be used as a bridge)
Star Screw Driver (for phone disassembly).
Plastic spacer tool (for removing the phone).
The first thing you need to do is clean the contacts of the GSM antenna and check, because most of the time or oxide powder becomes the cause of low or no network.