I'm struggling to figure out why WiFi is so slow on my pair of Deco AXE5400 routers. My son was complaining of lag and slowness on his bedroom PC connected wirelessly. Here is what we have done so far:

Moved his PC to the main router and hard wired it. We are on Spectrum Fiber 1GB speeds. Hard wired, download speeds were 500-600mbps. We unplugged the hard wire, ran the check again, 3-15mbps. I then checked with a phone and another laptop standing near the router. Same thing, 1-20mbps.

Next I checked router setting recommendations. I haven't really changed any settings since I bought it, but I verified that QoS was off along with other recommendations I found on this forum. I ended up not having to actually change anything because everything suggested to be off already was. Firmware is saying it is up to date.

Next I checked router traffic across all my devices. Not really much going on. A couple of devices were using data, but total usage was always less than 20mbps, and usually 1-2. I installed wifiman to see if I could find anything wrong (interference, etc). It couldn't find anything other than slow WiFi speeds. I installed the WiFi toolkit app and ran tests, which also found nothing. All the app tests just tell me everything is fine and here is your slow WiFi speed result.

After days of testing, the fastest speed I can get from my WiFi sitting right next to the main router on multiple devices is just over 100mbps with all other devices disconnected late at night. When wired in the same location I get 600+. The most common result is less than 20mbps. What do I do next?