I tried my SIM card ( a new one) on another phone an it works just fine. I'm wondering if I could be able to diagnose and/or fix this problem with expert advice from someone here. Thanks in advance!
Did you cut your card? The SIM card may be defective. Contact your mobile operator and ask for brand new card. The device slot may be defective too but it seems to be SIM faulty
Buy a new sd/simcard reader on ebsy/amozon for $6...install it yourself or take to repair shop. This is the final fix.
Watch youtube on how to replace SD and simcard reader for Samsung s4.