For a water damaged Nokia 2710, you must ensure to clean the PCB board first before proceeding any software and hardware repair. And for accidentally dropped units like it shows a dropping signal indicator on the display, you may restore and or update the flash firmware first before you attempting hardware repair.
If the phone does not being fix through firmware restoration, you may then proceed to do a hardware check up and troubleshooting.
The solution below shows which particular components that may need to check, trace and or then replace if found damaged.
Nokia 2710 No Network Signal Problem
Before you may proceed you must see to it that the antenna terminal contact expander were not damaged nor misaligned and it has a proper connection to the antenna module. A poor antenna connection also results to poor, dropping, and or no network signal receptions.
If the antenna connection is okay, you may then proceed to work on the RF Power Amplifier.
Check the RF PA chip's supply voltage, this voltage is feed from the phones battery 3.7 volts.
If there is no voltage reading, trace its line paths through to the battery connector positive terminal. Just manage to apply jumper wire on it if the line is cut, fot it is the only option.
If the supply voltage is okay you may then remove the RF amplifier chip. Check all the solder bumps if there is already lifted or damaged. Install a new RF power amplifier. In most cases a damage RF Power Amplifier result to No Network Signal like on nokia 2710 navigation edition.