As you say its not associated with any mobile broadband service, then all I can suggest is that you look at the help pages for BT branded hubs.
https://www.bt.com/help/broadband/getting-set-up/user-guides-and-manuals-for-bt-hubs
This is just a customer to customer forum, there is no specific technical assistance, but you may find a user on the EE forum who has this device, and can offer some advice.
