Looking for help with backup battery modem for landline.

Hello I’m in my mid 70s. I have my landline through Comcast whom I’ve been with for 35 years.  When deciding if I should go with Comcast for landline I was told that it will shift to a battery backup if internet goes down. I found out the hard way that’s contingent on having a new battery in the modem.  I’m not tech savvy but do need a new battery backup in case internet goes down.  My modem is maybe 6 years old. I don’t really remember.  I cannot talk to a tech support human because Comcast makes that impossible.  I’d like to find someone here who can tell me how a senior on a fixed income can get a modem with a fresh battery backup.  Thank you.