https://www.cnet.com/news/oomas-smart-security-system-adds-4g-and-battery-backup-ces-2019/

In addition to buying the 4G adapter, you'll also have to pay $5 per month for the Ooma 4G Smart Security plan. 

A lithium-ion power bank will also be available for $35 that's designed to kick in whenever the power goes out.