1. Apply and supply voltage to the PCB board using desired DC regulated power supply voltage of 3.7 volts. Check the supply voltage from the cellphone's battery through to the corresponding components highlighted in red color, these are battery supply voltage filter coils that being feeds to RF circuit components. IF there are no voltage readings on that areas, check the continuity of the battery positive terminal on that certain parts. Apply jumper if the line is cut or open. check also the filter coils and replace if found damaged.
2. Check and measure the secondary voltage output from the RF DC to DC converter chip highlighted in orange color. If there is no voltage readings on that spots, rework or replace the DC to DC converter IC.
3. Check or replace the Clock crystal oscillator. (VCTXO)
4. Rework or replace RF Power Ampilifier IC (PA)
5. Rework or replace Front End Module IC (FEM)
6. Rework or replace Radio Frequency IC (RF IC)