Issue with wired Ring 2 doorbell

My Ring 2 doorbell has been working for some time with no issue, it is wired into mains, but a few weeks ago we noticed it wasn't working, no lights when pushed or anything when I press the setup button. I took the battery out, charged it for a bit and the doorbell then worked again for a few days, then I noticed the same issue has occurred. Looking at the device health it says that it is offline, hardwired, and that battery level is 100%, I've charged the battery a couple of times and this fixes it for a couple of days. As it is hardwired and the device seems to be registering that it is hardwired and no loss of battery - what could the issue be?