Upgraded too 900mbps but speeds have dropped severely

[deadstar2020]

Upgraded the other day from 300mbps and whilst on the 300mbps all devices wired/wireless would show the correct download speed or very close to it. Since updating to the 900mbps package it has gone from 300 down too a 50-80mbps range which I cannot understand why it would go down on all devices. My pc which is the main device in the house is wired to a bt broadband booster as it is in my bedroom. I would just like some help as to why it’s now sitting at below 100 now I’m on the faster internet. My bt hub states I’m getting 930 down and 110 up which is the correct speeds but every device on speed test then shows under 100

Many thanks and any help would be dearly appreciated  

[deadstar2020]

And for more information the devices that are being used is an iPhone 13 pro max and a 12 pro max ( so as far as I’m aware capable of pulling in high speeds) 

then as far as Ethernet goes I’m using a cat 7 Ethernet lead so also capable of getting the speeds garunteed, yet it has dropped significantly and that’s worrying to me how it’s dropped dramatically since upgrading etc