Kobo Sage only works when plugged in - battery issue?

My Kobo Sage only powers on and functions when it's plugged into a battery pack or power adapter. As soon as I disconnect the charging cable, the device immediately dies. I've tried all the general e-reader troubleshooting steps, but the problem persists. iFixit does not currently list a specific replacement battery for the Kobo Sage. I'm looking for advice on further diagnosis, potential compatible battery sources, or any repair guidance the community might have.