Dual Sim iphoneXR - One IMEI is Blacklisted

A strange situation.

My late step-father bought my mother an iPhone XR 2.5 years ago. She didn’t use it and gave it to me. He is since deceased so we have no receipts and apparently no warranty.

Phone works great until last week my service provider (sim-only) migrated to a new carrier and I’ve had two weeks of fruitless trouble-shooting as I now have no SIM service. The phone only works on Wi-Fi. Whilst troubleshooting I checked the IMEIs (the XR has two) and one is blacklisted!

The phone is neither lost nor stolen but we can’t trace the original vendor. My service provider can’t find any explanation for the SIM not connecting and it was all working fine.

Niggling me is this: it has dual SIM capacity and IMEI2 is NOT blacklisted. Can I just get hold of a nano SIM and put it in the other side of the SIM tray to use the non-Blacklisted IMEI and (hopefully) get reconnected to my carrier?

Am I talking nonsense? It’s 1am and this is literally keeping me awake.

My carrier is a huge company and even their level 2 tech support haven’t a clue what the issue is.. we’ve tried everything but as I did not purchase the phone from them they can’t confirm or resolve the Blacklisted IMEI issue.

I don’t even know if the two things are connected - could it have been suddenly blacklisted from over 2 years ago?

Does anyone know if using a new SIM in the other tray to try and access the other IMEI could work?

I don’t know which way to turn.