wifi router issues

So i have the nighthawk router ax80 bought it back in august 2019. 

recently i been having slow download speed off my router to my PC(wired) i have 1000/mbps download speed and tapping out on 400/mbps from my router. now i have reset the router to factory settings and that would fix it for a while but not for long, as i did this maybe 3 months ago and now I have the same issue.

if anyone knows what can be the problem or anything else i can do to fix this issue.