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I expect the internal 5G radio card has failed, which others have reported on this forum. If you are still in a contract, then you should be able to get a free replacement, if not you would have to renew your contract.
If you are not in a contract, and do not want to be tied to a new 12 month contract, you may be better purchasing you own third party router.
Just ring the broadband helpdesk, they should be able to arrange a replacement.
The moderators are BT Employees, but they are very busy, and it would be much quicker to call the helpdesk, or use Live Chat.