It might sound daft but have you restarted it recently?
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Problem resolved. I have a BT model 6 router which allows the output frequencies to be split. 2.4GHz - perfect for when you're a few rooms away from your Hub - 2.4GHz will give you the best coverage around your home
5GHz - best for when you're in the same room or one room away from your Hub - 5GHz will give you the fastest speed.
I followed the instructions here