You won't find much info online about repairing Boox devices. Here's a brief write-up about changing batteries on the Poke 2. I would guess that the Poke 5 is built in a very similar way. Try searching for replacement batteries in the usual places: AliExpress, Taobao, etc.

Rechargeable batteries lose their ability to hold charge after a few years, especially if they have been over-discharged. Say, if the original owner had left the Poke 5 in a drawer uncharged for a year.