We ran into the same issue and had to redesign our boards to work with Pycom instead because I was told by Particle last year that they would not do the Verizon certification. They initially even expressed disbelief that I couldn’t get AT&T coverage. Talked to ublox as well and it seems to just be a matter of submitting IMEIs. At one point Verizon certification was even free, they just had to go through the process.
I would not recommend going to Pycom however. Their products look good on the surface but are not well thought out, missing a ton of basic features that you would need to do anything more than a basic example. Their documentation and support are some of the worst I’ve ever seen from any company. Their own code examples often fail to work or cause exceptions. They also have horrible stability issues that have caused us hundreds of lost man hours and data issues due to core panicks, modem firmware, and pycom firmware issues. The kicker is that their boards barely even work on Verizon either and seem to require different settings to connect based on what tower you are connecting to (same boards/sims that work in one location need to have the cid and other settings changed in order to connect after simply moving them a few miles away).
We were told by ublox to look into Verizon certification ourselves. But guess how that is coming along due to covid? We are pretty close to shuttering our business over this. We have lost a few clients and at this point have no solution.
It’s difficult at this point because we have older Borons running happily on Verizon for over a year now with no issue. The only problems we had initially with them were when using hologram sims and getting fplmn blacklisted. Switching to nimbelink on those devices worked great, but that doesn’t work on newer ones thanks to the IMEI issue here.