Hi I got a Deco x50 yesterday

I have 1.6gbps internet. I have prioritised device and checked QoS speed and everything. My wifi speeds will be 900mbps which is fine but only for like 5 mins then it will drop to like 10mbps for like 10 whole minutes! Sometimes it will climb up on its own, other times it will stay at 10mbps until I restart download. I'm wired with cat 6e wire to the 2nd deco in my room.. I'm downloading on a xbox series x.. tried steam downloads on my PC same problem although speeds seem to be stable-ish on my samsung s24 ultra on speed tests but lower than 900, usually like 600 on my phone which is fine but why does my xbox and pc keep dropping so low? Is there a setting somewhere that is causing fluctuations in my downloads?