So I will do my best to explain what's been happening and what I have attempted to fix the issue. First off, I have a g3 us990 under US Cellular running octos marshmallow (custom ROM) along with TWRP and have been for well over a year.
Last week I noticed I had no signal in my phone and a message on the lock screen saying invalid card. I have been able to temporarily fix it anywhere from 30 mins to a day where it then would stop and show the message again and texting/calling would stop working. I have tried restarting it(random if it would fix it or not), reinserting the sim card(found sometimes the no sim card is inserted error would show up), factory resetting(same as restarting), restoring from a back up(still nothing), and I have even went out and got a new sim card and activated it to see if that was it.
My next plan of action is to restore back to stock using the LG flash tool. But I feel that won't work either. Another idea of mine was purchasing a new sim card reader off eBay and doing some soldering, but I want that to be a last resort.
So at this point I am thinking it is the sim card reader in some way, but I would figure that if it was the sim card reader, it wouldn't work at all. I'm hoping someone that has had experience with this before knows a fix or any other ideas on what to do because I am just about out of them.