MK63 low download speed

Background:

I have Spectrum 400Mbps internet.  I used to have an old 10/100 Netgear router.  I have two desktops connected through ethernet and a few wireless devices.  The desktop used to get 80-100Mbps download speed consistently.

Today I received the MK63 Nighthawk Mesh WiFi 6 system and installed it.  I installed the Nighthawk app and check the speed on my Iphone.  It showed 400+ Mbps download speed.

I went to my desktop and check the speed, and to my surprise and disappointment, it showed 40-60Mbps.  My son told me his desktop showed 30Mbps.  These are connected through ethernet.  I'm extremely upset right now because I expected something close to 400Mbps, not 40Mbps.

Can some please tell me what the problem is?  I checked another topic about speed degradation and someone mentioned QoS being ticked.  I just the setting and QoS is not enabled.

If I can't get this issue resolved, then this MK63 is going back to Costco.

Thanks.