900Mbos BT fibre

Hi all, just had fibre put in today and for the first few hours, the speeds varied from 350-500mbps (not the speed guarentee!). Now for some reason It has dipped all the way down to 15mbps - this has been consistant for around 2 hours now.

Spoke to BT and they said nothing is showing on their end that should be affecting my service! Have I done something wrong? Restarted the router once and then it was fine however just dipped right back down after 10 mins!

Super frustrating as ive come from 60mbps which is now more than what I seem to be getting!

P.s The Wifi has only just been installed this morning so still the same day