Just tried to add APX320 to the wireless controller on the new UTM 9.7 beta.
I can see it leasing address from the DHCP server (with option 234 set to point to the LAN address of the UTM). However, the AP just finishes booting and ends up with solid red LED. I tried to a factory default reset of the AP with the same result afterwards.
UTM version 9.670-4