@khan, please note that the correct AT command to check for 5G SA registration would be AT+C5GREG?. However it would proably not show “registered” if AT+QENG="servingcell" shows SEARCHING.
If AT+COPS=? returns your network with <AcT> 12 and the SIM used is able to register to your 5G SA network when used with a non-5G RedCap device you might see a similar problem as observed by another RG255C-GL user here Missing enum? "Get RF Band Information" (0x0031) / "Bandwidth List" (0x12) -> Value 9 where the status was “LIMSRV” however.
You probably need the support of Quectel for obtaining Qualcomm logs like Catching logs from RG255C-GL with QLog to solve your problem.
Regards,
Reinhard