Thanks, yes that's exactly what it is then. In order to give external access to my devices (e.g. my NAS) I have to forward ports from their router to my router,

Thanks, yes that's exactly what it is then. In order to give external access to my devices (e.g. my NAS) I have to forward ports from their router to my router, and from my router to the device. So, definitely double NAT. Amusingly the person who installed it incorrectly called it a bridge. Thank you for the insight and lesson!