Not a lot of help, but I've been trying to set up autopay for 6 months as have a lot of other people and it doesn't work, we always get the "Something went wrong" message and no response from Comcast.
There seems to be some fault in their system that they aren't fixing.
I'm sorry to hear you've had difficulties setting up auto-pay! This is such a useful feature that we offer and I'd love to work with you to get it set up so you can enjoy the benefits! Please send me a private message with your first and last name, as well as your service address so I can assist.
To send a private message, click my name "ComcastTambrey", then click "send a message".