Have only been using cellular modem/router (Peplink) products since May, 2020. My observations, over that time, suggests the cellular providers are recognizing the demand for cellular data only plans & are gradually increasing the availability of these type plans. Kudo's to T-Mobile & Peplink in their partnership.
As it relates to 3rd party resellers of legitimate grandfathered unlimited data plans (GUDPs)..... my observation over this same period of time is that the initial fee (referral/setup fee) continues to increase, the monthly fees are now higher & the unlimited GB's are subject to use restrictions imposed by the 3rd party reseller. I suspect they are looking over their shoulder at the cellular providers with their data only plan offerings, future 5G, as well as, Starlink. I suspect they are trying to get their money now because a paradigm shift is coming.
Like all internet connectivity options, there is a finite amount of band with available at any given location..... regardless of how you connect. I doubt there is any ISP, fiber, cable, DSL, WIFI, or cellular that designs their network for peak demand. Wish that wasn't the case, but it's reality.
As for the FMCA Sprint/T-Mobile plan. I suspect they also block the SIM usage in Cradlepoint & other enterprise grade cellular modem/routers.
With that said, I've read of people running over 1700 GB a month through their FMCA plan & others complaining the coverage is weak to non existent. However, it the SIM would function in my Peplink device, without issue, I would certainly consider using it to supplement what I already have.
How quickly we lose sight of what internet connectivity was like in the '80's. Progress is wonderful, but, never fast enough.
