My phone died and when I turned it back on, I got a message saying that my SIM card was invalid. It's a Galaxy S6 that was originally on the T-Mobile network, but the guy I bought it from around 3 weeks ago unlocked it for me so I could use it with AT&T. Everything was working fine up until today. Any idea on why it happened and what I can do to fix it? Calling isn't an option, as I have no phone service because of this. I've already tried using the built-in T-Mobile app (it suggested I go into the app and use the Device Unlock option), but it won't let me do anything on it because I don't have a T-Mobile SIM card, I have no idea what to do at this point.
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