Hello Tanut Tigga,
As you mentioned the defective doesn't occur due to SIM reader damaged. you can start troubleshoot from GSM and 3G antenna and antenna contacts. visually inspect the area and see any damages on the antenna or contact pints. if yes you can simply replace the damaged antenna or clean the contact.
Visual inspection doesn't found anything mentioned above check the RF section of the motherboard and see whether any component missing, burned or damaged. also the fault can be occurred due to defective TRX or PA or SAWs