Thanks for the answers. Just a few
tweaks needed. The 'connection rate' isn't what you show (that is a speed test). What does your laptop show as its connection rate with the router? At the 10' line of sight, it should be at least 1200 and maybe higher than 2400 Mbps. Also, the 'Performance' mode I suggested is the Power Plan settings (or, put the slider all the way to the right), not the wireless adaptor card settings.
Use a Manual Control Channel and specify 160MHz Width (this last one may give you worse results if you're in a highly congested (wireless) area. Don't worry about which channel you pick, try them all and see which gives you the highest throughput.
Contrary to what may seem logical, you may want to try it at 15' and 20' distance too. Those 4 antennae streams need some room to converge and give the performance they are capable of.
As a guess right now, I would say you're connecting at 1200 Mbps or lower (go to Control Panel, Network and click on your connection to see that connection rate).
You may also be connected at 'only' 80MHz width too. (Here is where the 'Auto' control channel doesn't help...).
What laptop are you using? Are the drivers fully updated from the manufacturer's website? Is Windows fully updated? Is it plugged into the AC wall outlet during your testing? Does the laptop have two antennae (it should with that wireless adaptor, but checking never hurts)?
You can try using this (awesome!) software on Windows 10, below, to see connection rates, etc., but when you're doing any testing, this and all other wireless 'apps' and programs must be shut down for the computer to fully saturate the airwaves to the best of its ability.
Its best feature though is the Wi-Fi Channel Utilization it shows in real-time. A customer had a neighbor who had received a new AX router and they were trying to download the internet.
Finding a different Control Channel, even though their networks still overlapped, brought back my customer's speeds once again.
Helge Keck - WiFi Lite
If all the above doesn't allow you to see what the RT-AX88U is capable of, then I would consider doing a full reset to factory defaults
without importing any old saved backup config files, or '
blindly' putting in old settings that worked on your older router.
You may also try using a new SSID as a quick test (use one of the Guest networks to do so) to see if your computer and/or phone are 'remember' any microcode and limiting your new router's capabilities in some way.
New M&M 2020
Fully Reset Router and Network