After changing SIM, can't receive service

Hey all, I went abroad to Japan for a couple months and swapped out my SIM for a Japanese data SIM card which worked perfectly fine. However when I came back to the states and inserted my old SIM card, it didn't work anymore (SOS only)

I talked to my carrier and did the SIM swap test and the issue is not with my carrier or with the SIM card, as my SIM card works perfectly fine on other peoples' phone, and other people's SIM card does not work on mine. Did a network reset as well...

I know my phone model is "old", and the phone itself is almost 5 years old, but I find the timing of it really suspicious and am in disbelief that it suddenly doesn't work anymore just as I get back to the states. Is it possible that there's another factor contributing to why it doesn't work? I really didn't want to replace my phone right now...

Is probably unrelated but I used a lot of memory in these past couple months taking videos. Still have around 5 GB/256GB free space though.