I had the same problem earlier this month, the problem has been resolved I uninstalled the app then installed again using my BT password, all up and running,I dont know if this will help. I have an amazon fire box thats a few years old.
I tried this but BT tv not accepting my BT password. Seems I need to add BT tv to my package. I already pay for broadband, sports and mobiles. Need a lottery win for all these addons
I have got BT Landline BT TV and Broadband with Bt Sports but not mobile, like you say it costs an arm and a leg but keep trying, I think when I had the problem there was an issue with a lot of customers, so your issue may be different.
Hope you get it sorted.
See I don’t have BT tv as I’ve sky Q. I’d like to access app without having to take a BT tv box. Hopefully get sorted.
The BTTV app on Amazon is a BTTV add on only. It won't work without one.