This is a big deal and very frustrating. It looks to me like Sprint subverted the unlock law of 2015 by setting their bootloader to v6 and not allowing it to go backwards. I've filed an FCC complaint and received multiple calls from sprint about it. I would urge you to do the same. Now I have to sit and wait for TM to update to a v6! Bootloader so i can use their service correctly. Again, this isn't TM's fault outside of the sales people not telling me wifi calling and other options won't work with a sprint phone.
Hopefully this will be resolved soon, but for now my TM service is useless.
That has nothing to do with unlock law. FCC will do nothing because changing the software has nothing to do with unlocking the device.