Have you tried a factory reset? Hold in the pinhole at the rear of the router for at least 50 seconds and then release (the lights will flash while holding in the pinhole reset button after about 10-15 seconds) then allow 5 minutes for the router to reboot and re-sync the connection.
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BT Full Fibre 900 FTTP - Asus RT-AX82U to Openreach ONT Box & Asus RT-AX56U with AiMesh setup
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