Rain refusing to cancel service

Last year between July and October 2023, someone stole my (Rain product) router with a 4G sim card. I contacted Rain within 48 hours of realizing it was missing, hoping they could track it and stop its use. They confirmed it was still active, so I asked them to immediately cancel the service.

Despite multiple calls - two to three months later and again in December 2023 - the service wasn't canceled until that December. Rain kept charging me from August to December 2023, refusing a refund, claiming the device was in use. This caused issues with my other 4G internet service, which Rain also wants to charge me for despite my requests to cancel.

Trying to resolve this has been frustrating. I've been calling Rain multiple times a week in December 2023 and even today (January 3, 2024), but there's been no solution to my problem. Any advise on what to do with a service provider that holds you hostage?