I’m not sure Astell & Krell portable music player is Roon Ready. It requires a Roon codes embedded inside the firmware in order to work. So far I’ve not heard any portable music player in the market that is Roon Ready.
However you can use iPhone and connect to AK320 as DAC but Roon only see iPhone not the DAC.
Roon cannot see the DAC directly but only the iPhone. Since you can use Roon and stream to the iPhone, the iPhone should output digitally to an external DAC via its lighting port.
My suggestion is can you playback using just the iPhone to your AK320? If you can’t, then the AK320 DAC is drawing too much current via the OTG cable from your iPhone. If you have an iPad try that. I use my iPad to my FiiO X5 mk3 music player as a DAC and it works fine
Even though Macbook is running as endpoint, the Roon software you installed in MacBook is still considered a full version capable of switching to Roon core anytime. The iPad and iPhone version is strictly a Roon Remote.