Need help using AT&T v3xx on T-Mo USA

Hey all,
I’m trying to get a Motorola RAZR v3xx from AT&T working on T-Mobile USA. 😎

I thought it would be pretty sweet to use a v3xx as a "retro" 3g-capable phone on T-Mo. I’ve used so many random phones in the past that I thought it would be easy to set up…. but now I’m starting to think that it may not be possible. I would love if somebody could chime in their experience.

Here are (more or less) the things I’ve tried so far:

1. Bought the phone on eBay – locked to AT&T
2. Bought an unlock code on eBay
3. Phone would freeze during AT&T start-up sound when active T-Mo SIM was inserted, so I couldn’t enter the unlock code as normal. It may even go to a critical error screen.
4. Put an old, inactive AT&T sim back in the phone. Some functions work with this SIM. Used RadioComm (MA=GSM) to get SBSDY_LCK status – got 01 (locked)
5. Used RadioComm to enter code and check SBSDY_LCK valid – got success
6. Used RadioComm to check SBSDY_LCK status – get 00 (unlocked)
7. Put active T-Mobile 3-in-1 sim back in the phone – still crashes.
8. Looked for the next option – flashed a new firmware using F&B: S1mPl3 V.2 for V3xx MLD (ZIP format) – Downloads – *****Mobile Forums
9. Put active T-Mobile 3-in-1 SIM back in the phone. It registers on the network, then freezes shortly thereafter.
10. Put old, inactive T-Mobile SIM in the phone. The phone starts without freezing and some functions are usable except network (leading me to believe it is truly unlocked).
11. Put old, inactive DoCoMo SIM in the phone – some functions work/no freeze (also seems unlocked).
12. At this point, I think the 3-in-1 SIM might not be readable by this phone. I called T-Mobile and asked:
a. if there are any known issues with the 3-in-1 SIM not working in older phones (from my experience it works fine in old LG devices). They said no.
b. if my old, inactive T-Mobile SIM could be reactivated. They said no, the number has been recycled.
c. if T-Mobile sold any non 3-in-1 SIMs that I could try. They said no, they only sell 3-in-1.
d. if I could buy a non – 3-in-1 T-Mobile SIM from Amazon and activate that on the network. They said maybe. I figured I would give it a try.
13. Bought a non-3-in-1 (expiration date 2017) T-Mobile SIM from Amazon and activated it on my account.
14. Put the new non-3-in-1 SIM in the v3xx. It registers on the network, then crashes, just like the 3-in-1 did.
15. Called T-Mobile again to reactivate my 3-in-1 SIM/ deactivate the new non-3-in-1 SIM.
16. The new deactivated non-3-in-1 SIM still crashes the phone (so it doesn’t seem network related – it definitely seems SIM related!).
17. Went to at&t store, where they put in a test SIM and placed a test call, proving that the device works.
18. Went to T-Mobile store, where they had no clue why I’m trying to get this device working in 2016 . They suggested that "the newer sim cards are formatted differently" (whatever that means), so 4g- sims may not be backwards compatible. Across the web I saw some post about SIM voltages but nothing definitive about this model.

Bootloader: 03.08
SW Version: R26111MLD-AS_U_96.B0.0AR
The stock firmware on the device was: R26111MLD-AS_U_96.B0.0FR

As you can see, I’ve crawled through the forums and tried out several options to narrow down where the problem lies. :confused::confused::confused:If you have any pointers, please share!! I’d also be interested to know if this has happened with sims from other carriers.

Thanks in advance for your help.