unfortunately i don't have access to my AX9000 until Sunday.
The ax6 method is also described here:
basically it's using the extend_wifi api on the ax6. As far as i understand, you setup a second wifi-Router with a special xqsystem.lua file (including the nvram commands) and then connect to that router via wifi and the ax6 reads that file during the wifi connect.
And those extenwifi api's are also there in the AX9000 firmware. So i think it's worth a shot.
This is the URL to connect to the others router wifi (from the ax6 or AX9000:)
http://192.168.31.1/cgi-bin/luci/;stok={STOK}/api/misystem/extendwifi_connect?ssid={WIFI_NAME}&password={PASSWORD}