Hi

Is it possible to substitute the supplied USB WLAN-adapter with a 802.11ac-compatible adapter?

The Edimax "dongle" is not proprietary to Axiom (you can buy it elsewhere) and there is nothing special loaded on it to make it compatible with Axiom gear as far as I can tell... It merely makes non-wifi devices wifi capable. Since 802.11ac is backwards compatible to the Edimax's 802.11b/g/n, it should work without problem (though overkill as you are not streaming 4k video... or are you?) :-)

My AxiomAir seems to have problems connecting to my network after I have added an AP. I think it could be because it doesn`t support 5ghz. If I turn off the AP the AA works, if I turn it on the AA blinks after booking even if I reset it with a USB-stick.

And I just love my music in 4k HDR 😜

When you say "added an AP", do you mean that you currently have a Wi-Fi gateway configured and now you have two? If so, make sure that they are not BOTH configured as DHCP servers. Otherwise you will have strange and inconsistent problems a little everywhere in your network.