First check from software level to fix your fault,
Settings--> General --> Reset --> Reset Network Settings
Or check with " Erase All Content and settings" Note: All data will be lost on the phone
If the fault couldn't be solved in software level you have to
Check any sim contact missing or bent
- As you mentioned if the LCI triggered you should have to clean the area with isopropyl alcohol
if non of above could solve the fault you may have to replace AS3922 chip to fix the fault (Sound BGA reworking skills required) in this step if any mistake you made, few functionalities wont work anymore including NFC
Officially, you should required to sim card to activate your phone.
Hope this answer help you to fix your phone by your self.