Ring Video Doorbell 2 has switched from wired to battery and stopped charging

My Ring Video Doorbell 2 had been hard-wired for around 18 months without fault. Within the last 2 weeks I started to get batter low messages and in Device Health it now shows the Power Source as 'Battery' where it always used to say 'Wired'. I've had the power supply and connections checked and it's still receiving ~15V from the transformer (Honeywell D753 8-24V). The installer noted this is the same reading as when he first connected the device. I've 100% charged the battery a couple of times now but it won't return to the 'Wired' reading.