Recently my LG-P659 phone, originally from T-mobile, has had a lot of apps crashing or exiting for no reason, and today it can no longer get past the splash screen on boot-up (if it gets even that far). When it gets to the T-mobile splash screen it ends up in an infinite loop at said splash, and sometimes it just ends up in a black screen.
This phone was originally purchased in the USA by a family member, through T-mobile, but I am currently living in France for a few months and am using a sim card for a local carrier here called Free, using a month-to-month plan. The phone was unlocked from the T-mobile network through a code provided to me by T-mobile. It is unrooted. The phone is no longer on an account with T-mobile, though the account it was originally associated to is still active (but I am not the owner of this account).
I attempted a hard factory reset, but all this results in is the error message "Secure Booting Error! Cause: boot certification verify". I managed to get it to boot up once after that, when trying to get it to start in Safe Mode. Everything was reset, but I made the (apparent) mistake of restarting the phone after getting some things updated and reconfigured, and now it's back to being stuck in the boot loop. Repeating the previous things has not been able to get me past the splash loop again.
So the question is: who am I supposed to call about this? T-mobile, Free, the phone manufacturer, or someone else? And do I have to call a French line or an American or what? Making the call itself will be hard, seeing as my phone is dead, but I think I can manage to solve that issue. And is there anything else I can reasonably try on my own, or should I just leave it as a brick for now?