This is entirely your own problem, and in no way a DV ‘fault’ , you could plug your phone directly into the router it will work properly,
if you have somehow cobbled together your wired extension sockets to be connected to the router and it’s been done incorrectly that’s your issue to solve , if you don’t have wired extensions but you have requested DV adapters and where you have them located is out of range of the hub ( this would be exactly the same as regular cordless DECT phones being too far away from their basestation ) , they obviously won’t work .
If you need existing extension telephones around your property to work , and a cordless DECT phone (or DV adapter which is just a DECT adapter) won’t work because of distance, then you need to wire the extension sockets correctly to the BT Hub ( and disconnect them from the main socket on the wall ) if this is beyond your DIY skills, then you would need to employ someone to do it for you , extension phones are not BT or Openreach’s responsibility but yours
