You can effectively do this with the AT&T 100 GB for $55 data plan. It's prepaid so there's no obligation to pay every month.
https://www.att.com/prepaid/mobile-hotspot-tablet/
if you don't pay again (i.e., "refill") when the month ends, it stops working but you have some amount of grace period while the account and associated phone number remains active and all you need to do is pay another $55 to bring it back to life. The grace period matters to some people on a prepay phone because they don't want to lose the number but that doesn't matter for a router. Even routers have a phone number but nobody cares about it.
Nobody seems to know how long that period is. For prepaid phones it seems to be 60 days but for this data plan it seems longer. I bought it for my Pepwave, I think it was in April last year. I used it for a month to test it but didn't refill and haven't touched it since then. If I login to the account now it still shows the number and a big prompt for me to pay another $55. I think if I did that it would work again but I've moved on to something else so I don't want to do that just to prove it. I think the grace period might be a year.
But even if that doesn't work out, there are two other options. The simplest thing is to keep a blank AT&T SIM on hand (costs less than $5 on Amazon). If you wait too long and it expires then just create a new account here
https://www.att.com/prepaid/activations/#/activate.html. That might cost an activation fee. I think it's $15.
The other thing I guess if you don't want to risk it expiring then you can login to the account and change the plan to the $35 per month for $15 GB just to keep it alive. You definitely get 60 days grace so at a maximin you could pay $35 every three months. One month makes it work (even if you don't use it) and then two months grace period.