Then you should contact your phone's carrier about issues with your phone.

I think your phone is either modified/rooted or someone flashed a different region firmware on your phone without charging the csc. You can search your phone's name and model code on SamFW/sammobile and update your phone manually.

Note 10 plus won't get OneUi5 (A13) update. Only the lite model will get the update.

I'm not sure. I changed mine to BTU a few days ago, and I didn't receive any warning or something about it like you do when you root your phone.

Make sure to flash the firmware after changing the csc.