The solution below shows the RF circuit on Nokia X3 PCB board layout.
You may need to check, rework or replace these components for further no network signal problem troubleshooting.
Nokia X3 No Network Signal Solution
You may start to work with the RF Power amplifier (PA).
Try to replace it with new replacement one but before that check the RF supply voltage first which came from the battery voltage 3.7 volts.
If the power amplifier has been already replaced but nothing happens, you may then try to rework the RF chip, or replace it if necessary.
There are also some possibilities that the signal lost is caused by a faulty or damaged oscillator and RF filters.