I was able to get this working again and I wanted to report back here with my findings.
For a little more information, I am on the “AT&T Unlimited Elite” plan.
I called the AT&T support number and spoke to someone who seemed very knowledgeable. They told me that once AT&T flags an IMEI is a 3G-only device, it essentially automatically gets added to a blacklist and a “data-block” & “call-block” get put on the SIM card ICCID. This means the SIM card will not work even if you put it in a new device. The SIM card will maintain this block until you call support and have them remove it. There is no other way to get the block removed.
However, if the IMEI ever attempts to register to the network again (in this case the X750v2), the blocks get re-applied to the SIM card. This is true even after upgrading to the latest firmware, as it is tied to the blacklisted IMEI.
To fix this, I activated the SIM in my phone (by calling support and having the block removed), moved it to an old phone I have that I no longer use (Google Pixel 3), and let the Pixel register to the network. After that, I turned off the Pixel, turned on the X750v2, and changed the IMEI of the X750v2 to match the IMEI of the Pixel using the following AT command:
AT +EGMR=1,7,"PIXEL-IMEI"
Once the X750v2 had the matching IMEI, I turned it off, put the SIM in the device and turned it back on. I also made sure to match the APN with the one in the Pixel, which was nxtgenphone
.
I am under the impression that the original IMEI of my X750v2 will never again work on AT&T due to the blacklist.
I hope this helps someone else in a similar situation, although you will need the IMEI of an old unlocked phone that supports 4G LTE on AT&T to make this method work. Also, based on what I’ve read online, you probably don’t want your extra phone (Pixel in my case) to ever attempt to get back on the cell network since it can “confuse” the cell towers. I’m sure there’s a technical explanation, but I didn’t research it much further than that.