Years ago Verizon had an unlimited talk + text + 3g data plan for $15/mo. It's still grandfathered and the accounts can be flashed onto modern phone and hotspot hardware.
If you don't care about the phone + text part you can add a $5 data refill every month and the 3G data will keep flowing. When the full $15 accrues in the account the plan officially renews. Until then, the prior two refills will result in Vz saying the plan was not renewed but data still works. The account can be closed after 90 days of non-renewal. This means if you have been paying only the $5/mo and forget to add the third refill in time you could lose the account and render your hardware useless.
Those depending on this plan as a primary (or only) source for mobile data might consider paying the full $15 each month. Even safer would be to set up $15 autopay on the Vz site, although sales tax may be charged.
Callingmart.com is the place Howard Forum phone gurus get their refills.