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Is your BT Fibre new? If so, check for parental controls.

Also separate the 5ghz channel that can help as well
 
Plus when you make a 5ghz in your router it is much better if you change a different name than the 2.4 name. What I did was just add 5ghz to end of my 2.4 name to separate it from the 2.4 name. This way I could know what my devices were connecting to my router.
 
What Apple hardware do you have specifically? Can you describe your network setup?
 
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