By default, there is no option for 802.11ax in IPFire (yet), you have to edit the WLANAP.cgi page or configure hostapd manually.
I wrote a patch for the wlanap.cgi-page a couple of months ago
for standard-802.11ax and 802.11be compatibility, but you have to check if it fits with the mediatek-features (especially concerning the encrypting ciphers). And then the new IPFire-version (188) will bring an revised wlanap-page so the patch won’t work with this one anymore (but I will write a new one for myself and can post it then).
Greetings
Alex