Asus xt8 mesh system with Main router Asus gt ax11000

[thekingz]

Hi I'm new here wondering if someone can help me put what it is I have got the Asus ax11000 router and I have connected the ai mesh system with the Asus xt8 as I've got a large house so need the net there aswell. The problem I have is the speeds on the xt8 is some times fluctuating in speeds the main node which I have is directly wired into my main router and the other node is upstairs in one of the rooms I am getting really good signal no issues there its just the speeds I pay for 600 mbps broadband and upload is about 42 which is fine on wired connection but I won't get that full 600 on wireless it will be about sometimes 280mbps and sometimes 420mbps download speed don't know why that is. Am I doing something wrong hope someone can help out and sorry for the long post.