For Verizon, have you tried starting here? Select “other device” and hit next. https://www.verizon.com/onedp/byod#/
That reports my Pepwave MAX BR1 MK2 is compatible. I don’t know how I even got to that page now. The site is so confusing I usually end up going round in circles getting nowhere.
If your device is compatible then you can move through the badly designed and confusing process to eventually get to choose a plan. Unfortunately it doesn’t help much if you want any significant amount of data without paying a fortune. They look promising but I think the router only gets the hotspot amount before being throttled to 2G speeds. That’s only 30 GB on the highest plan at $85. At least that’s what someone on ltehacks.com told me when I asked about this when I thought I’d found something usable.
For what it’s worth Visible, which is on Verizon, seems to work reasonably well in routers and I haven’t seen any reports of being shut down for device mismatch. I use it for my phone and temporarily put the SIM in the Pepwave and it worked quite well. You need to select “Generic” in the settings, not “Verizon” and I think the APN is VSBLINTERNET. Unfortunately, Visible speeds vary a lot from really good to almost unusable and latency is high but it can be a good deal at $25 (with party plan).
People also report success with Page Plus phone plans which is also on Verizon. They seem to have an unofficially cap at 200 GB.
For AT&T the “Unlimited Elite” postpaid phone plan seems to work, at least for now. There’s also the $20 postpaid tablet plan that is very popular on ltehacks but it seems that you really need to spoof the IMEI as a tablet to be safe with that one which I don’t think you can do with a Pepwave and is probably illegal although nobody on ltehacks seems to care about that.
The scary thing for me doing this sort of thing is not so much getting shut down but getting the IMEI blacklisted which makes the router an expensive brick on that network. I haven’t heard any specific reports of that happening but I think it could.
Sorry to be less than encouraging. I think the consumer market for this is so small compared to the phones that the carriers are not interested.
(I wonder if we connected on irv2.com. Ignore if that makes no sense.).