BT DV adapters are not WiFi devices , they are DECT , same as any ordinary cordless home phone , all the adapter does is make a corded phone into a DECT handset so it connects to the built in DECT basestation within the BT hub , its intended audience is those that claim they have corded phones no where near the router so can’t simply move the cord from the wall socket to the router socket .
Changing BB providers won’t make your handsets connect to WiFi .
Try resetting the BT hub , and then with preferably a corded phone connected directly into the BT router phone port make an incoming call to see if the phone responds.
Its not clear if you have removed the BT router in favour of a third party device , but if you have then clearly you won’t have use of your BT supplied telephony, because that requires you to use the BT supplied router .
