Why can't I receive incoming calls on my unlocked LG G4 phone while abroad?

[AC Question]

I was overdue for an upgrade, and recently got the LG G4 smartphone. It worked just fine while I was in the States. Sprint also unlocked this phone for me because I live and travel abroad. I have about half a dozen SIM cards for all the different island nations at which I'm currently located. The SIM card for my current location works perfectly in the old phone, so it is not the card. It is not a dual SIM phone, only one SIM at a time can be put in. I can make outgoing calls with no problem. I have been through all the settings what feels like 100x, trying to see where I could have gone wrong.....but have had no light bulb moments. Thoughts? For the record, I am not tech savvy. I have spent most of my time in 3rd world countries for the last 5 years and technology has left me in the dust. So please, try to keep things dumbed down a bit for me.