I’m surprised you had an issue with Tmobile. They seem to be least restrictive.
If you are ok with Tmobile, one surefire way is to just go to a Tmobile store and buy a Data-only SIM with X-amount of GB. They do not ask for, nor validate your IMEI prior to selling you this sim in the physical store. Once you have the SIM card the tmobile account web pages for managing that SIM card do not even show IMEI. They really do not care what you have it seems and will gladly accept your payment.
ATT & Verizon on the other hand apparently dont want paying customers and block you from signing up… over claims of “compatibility” which is BS. This is a carrier issue in my opinion. They want to play games to drive people into buying hardware directly through them. I think they are also trying to squeeze money out of hardware manufacturers to “certify” devices which is nothing more than whitelist all the IMEIs, for a fee.
However the signup pages for ATT & Verizon can be fooled by entering in any “valid” IMEI, for example if you could “find” an IMEI for a Verizon Jetpack…the web page will accept this even if its not what you will be using. Certainly there is some risk with this approach but it works. I have found that once they detect the IMEI it no longer allows changing the plan because no plans are “compatible” with the detected IMEI… but i dont care, I can just cancel and resignup on another prepaid plan when and if the time comes.