How can I get this former Lifeline phone working?

I have a UMX U683CL. My mom has had some of the worst luck with phones via Assurance Wireless; this was her third or fourth one in about three years, and they've all misbehaved terribly. They didn't even want this one back, and I found it again recently and thought I'd repurpose it as a much newer phone for me (my phone is a six-year-old Alcatel potato that's served me well running apps, but it's small and slow and old). I started down the road of rooting and got super overwhelmed and confused in a hurry, and ultimately gave up and just used Tiny ADB to uninstall all the bloatware on it that I didn't need. It seemed like it would be fine, so I went ahead and got set up with Tello and had them send me a new SIM card. It got here, I put it in the phone... and nothing. I ended up doing a factory reset to see if that helped, since it could just reinstall all my apps and things from the cloud easily, and still nothing. All of Tello's advice involves updating the system in various ways, and that's where the problem is. This phone never succeeded at updating once, in the time my mom was using it. It would periodically try and fail, and was mostly an annoyance whenever it did. But if I do Settings > System > About phone > System update (Update OEM software), none of the options there succeed at anything. I do update profile, it makes a cursory attempt to do it and then I get a popup about "waiting for retry" (with five attempts). Each attempt takes a minute; if I let it muddle through all five, eventually I get a message that says "Your activation could not be completed. Please try again later. If the problem persists you may need to contact Customer Service." I do update PRL, and the mU Software Update overlay just sits on "Checking for PRL update" infinitely. If I go to system update, I get a screen with info about wireless updating and how it works, and if I hit "checking for updates", it checks and then says no updates are available. If I hit the last option under the system updates menu, UICC Unlock (important for this instance), I get a popup from mU Software Update that says "UICC unlock fail. Error Code: 23. Description: DM Auth failed with 407 status code". Assurance Wireless isn't going to be able to help me with this. I don't even want to bother with calling them, because I'll end up with someone in a call center in Dubai or wherever, and they're not going to be able to do anything with the fact that it's not a current, active Lifeline phone. So. Is there a way to deal with this? Would rooting get me around these issues and let it work as a phone NOT on the Sprint network? **Edit:** Since I've mostly struck out with this same post over on Tom's Guide, I'll clarify some things here as well. My primary wish is to get the update functions working so that I can progress with activating the Tello SIM. As far as I can tell the phone is already unlocked; Assurance Wireless will likely be of no help, and my mom never dealt with Sprint directly, because this is a former Lifeline phone, so they wouldn't be able to help either. And if the updating can't be fixed, I would like to know IF rooting would bypass the problem, and if it would then I have to find someone who can really hold my hand through it because my dip into rooting was super baffling and unsuccessful.