I used to have BT Broadband FTTP in a previous house, which had a bidirectional fibre into a box of some sort, which was battery powered, and went into another

I used to have BT Broadband FTTP in a previous house, which had a bidirectional fibre into a box of some sort, which was battery powered, and went into another box which emerged as a telephone jack, and an RJ45. This was 2016. The battery box was there to provide power and service for a landline in a power cut. My router plugged into that RJ45 with a pppoe client, username "bthomehub@btbroadband.com" and looks like any old password ("BTSux" for example)