Update** so I switched my nighthawks off and tried to relink and it worked first time. Thank you for your help 👍🏼
That infers the BT router is not in Bridge mode or router-only mode at the moment... You might want to look into that further, that’s if you plan to re-instate the nighthawk wireless mesh system.
Your BT WiFi would be switched off in either of those modes and you can resort back to using the nighthawk Hubs for your network and Home WiFi instead, which may provide you with better Home WiFi coverage (perhaps?), but in that situation, when using mesh WiFi, it’s often best to wire a single Sonos product (not a surround, or Sub) to your primary nighthawk mesh Hub/Router and run your Sonos products on their own SonosNet signal instead.
A couple of other things that goes with that SonosNet setup is to remove your WiFi credentials from the Sonos App network settings, keep the wired Sonos device at least one metre away from the wireless Hub and do not cable any Sonos devices to the nighthawk satellite Hubs.
It’s often helpful (but optional) to put each of the Sonos devices IP addresses into the routers DHCP Reservation Table to ensure that they get same IP address on (re)connection to the LAN after update/reboot etc.
Hope that info. assists.