Router limitting my speed??

High and good day to you guys.

I'm having problems with my router
I just bought it yesterday and it was actually working perfectly fine
I have spectrum for my provider and I have 400Mbps plan

I tested it with their modem directly plugged in to my pc and it was working great
But then I needed wifi so I bought this router.
Note: I checked the speed and run a speedtest after I installed the wifi router and I was getting the correct speed which is 400Mbps

But then again today when I run a speed test again just to check is everything is working perfectly fine, it seems that it caps me out to at least 90Mbps

I called spectrum first, they run a speedtest in my modem, then we tried plugging the pc directly to the modem and it was actually working perfectly fine. Again 400Mbps

But when I plug it back in to this wifi router (Nighthawk AC1900)
it goes back to 90Mbps

I actually don't know what is happening and do you think I need to change settings in the router itself?
I am calling tech support for netgear Right now, but Ive been on hold for at least 1hour and 25Mins now.