Do you have a BBU, (battery backup unit)?
If so, I suspect the batteries have died. The BBU is no longer supplied so just bypass it and plug the power supply directly into the ONT
Not a problem, open bottom door, remove screw on right side and open top door, remove power lead, push battery box up and remove, replace the 4 AA rechargeable batteries with new ones and replace battery box and and power lead and close all doors. Red fault light is off and charging light is on. Job done.
Most likely the BBU itself that’s faulty.
They were the cheapest of the cheap, even Poundland would probably struggle to sell them off on a Sale.
When I was tools I must’ve replaced at least a dozen a month.
Unless you have FVA, which I’m 99.9% confident you don’t as I’m sure Openreach have completely withdrawn the Service you can just remove the AC Power Supply from the BBU and connect it directly into the ONT.