BT Broadband Routers are cack, and start to disconnect for no reason.
May be a dodgy phone line, but unlikely.

I would go with either a setting, as other comments, or modem dying.

I swapped to a bought-in router, rather than the BT one for the same
problem & it has workied fine since then.
(my BT one was a small blueish black box, with abnout 3 green lights on
the front)