let’s try some solutions:
Cleaning the slot:
- Remove your battery
- Remove your SIM
- Using some types of textil, try to clean up slot carefully! If it’s possible, use PCB Spray to clean it. Also, check if the pins are OK without any visibly damage (compare with a new slot) or corrosion.
- Insert your SIM and boot your phone
- Turn OFF your phone
- Press POWER UP and VOLUME DOWN buttons together to enable fast-boot option
- Using volume buttons, navigate into the recovery menu
- Choose “wipe-cache partition”
You can also make sure, that SIM card didn’t work in safe mode or airplane mode. You can check, if your phone did not change settings of preffered network like LTE, CDMA, GSM .
The hardware issue is preferred - start cleaning, CAREFULLY!
Let me know, if it’s ok or you need another solution.