Data-Only Plan Stuck in Broadband Portal

I have a data-only plan through a reseller, so I haven't been able to get this resolved myself with Verizon support, but I was wondering if anyone could shed any light on the situation.

I have a 3rd party modem that takes a VZW SIM card. Up until this weekend, everything worked fine. Suddenly, over the weekend, Verizon started forcing me into this captive portal:

(I've removed my IMSI from the link). It instructs me to "Click continue to verify and connect your device."

When I click "Continue", it goes to

The message I get is "You device is already active." However, my internet access is still denied, and I'm stuck in a loop with these screens.

Looking into the page source, this error is called a "Prepay Acct Maint" error. Any ideas on what this means?