Sprint LS997 unlocked running 7.0, can't add APN

[nookin]

I have this LS997 that was unlocked. It was given to a relative. It had been used on T-Mobile for a while.

Sometime around last week, it stopped working. I checked the phone, it has not been updated since I last updated (still at 7.0 with Jan 2018 security patch, I didn't took the Oreo update because I know it will lock the phone again). But the APN section is now empty. The ". . ." menu is accessible as before. I tried adding an APN but after clicking "Save", it does not show up. Also, I noticed that the T-Mobile APN that I previously added briefly show up after reboot. But quickly disappeared.

Any idea how it could happen?

 

[nookin]

Answering my own question.

After some troubleshooting. I noticed that the "Secure Startup" was turned on (request PIN on boot). Once I turned it off, the APN shows up again and I can once again use it with T-Mobile. No ideas why they are related.