This router does not support Wi-Fi 6E (I believe you're asking about Wi-Fi 6E - extension of Wi-Fi 6 (802.11ax) that operates on the 6 GHz band).
Asus TUF-AX6000 is a Wi-Fi 6 router that operates only on the 2.4 GHz and 5 GHz bands.
Did you set the "Country Code" in your Wi-Fi router settings?
When enabling Wi-Fi, it is mandatory to set the correct country code where your router is located.
Wi-Fi 6 (802.11ax) as the successor of Wi-Fi 5 technology (802.11ac) does NOT imply that it operates on the 6 GHz band just because it includes the digit 6 in its name.
It is advertised correctly as a Wi-Fi 6 router (802.11ax). For 6 GHz band you need Wi-Fi 6E or Wi-Fi 7 router.
According to the published specs, your iPhone 15 Pro Max is a "Wi‑Fi 6E (802.11ax) with 2x2 MIMO" device. Getting only 400 Mbps on a speedtest is definitely not normal, assuming that your WAN connection is faster than that.
Having said that, the default wifi-device config is correct (and must say 5g for a 5 GHz radio), except for country codes.
I suggest trying a different (hopefully less congested) channel. In most countries where channel 100 is valid at all, it is worth trying.
try changing the encryption to psk2+ccmp
also "2401.9 Mbit/s, 160 MHz, HE-MCS 11, HE-NSS 2" is a very good result so the router and phone have a good connection
That's because the kernel understood that it cannot use 160 MHz and automatically narrowed the bandwidth down.
Have you tried both channels? DFS channels are usually more clean and permit higher speeds.
Another useful metric to get is the signal level (displayed on the Network > Wireless page). E.g., in the example below, the phone is heard by the AP with the strength of -52 dBm.
tried 52, 64...
theres something strange that it fluctuates on 24.0 Mbit/s, 20 MHz every couple of seconds..
not stable..is that normal?
the dBm is between -52 & -60 - im just next to the router almost.