Ring doorbell transformers

I was asked to install a transformer to a ring doorbell 2 with battery. I purchased the transformer from ring. There was no doorbell or transformer installed so I have come straight off the main lighting circuit to an outside weatherproof consumer unit housing the ring transformer inside. I then installed a small cable from the output of the transformer to the ring doorbell. The wires were connected to the terminal screws on the back of the doorbell. Everything had been working fine for about 6 months. I then got a call to say the door bell has no power. Now I understand that the mains supply only trickle charges the battery. I then purchased another transformer from Ring and installed it. This worked for about a month and failed again. I have spoken to Ring and they assured me this should work fine and I even asked if I needed to installed a resistor but they said no. Since then I have purchased another make transformer (the correct type) and it has been working fine for 3 months and now the same again. To be honest I’m sick to death of it. The instruction manual contradicts what Ring tell you. The manual says install a AC transformer yet ring supply a DC transformer. I believe that clearly the ring doorbell with the battery isn’t meant to be charged using a transformer. It clearly doesn’t like it.