This one is a bit tricky because it’s not documented and few post i read about that were people trying to make it work. I do have SFP port on my switch but only 2 and the last one available will be use for NAS 10Gb access.
In order to achieve this link aggregation you need to make sur your switch support LACP, my Unifi USW-EnterpriseXG-24 does.
You need to configure the LACP on your switch before configuring the SDX Channel. For instance, i’ve aggregated port 23 and 24 of my switch.
Switching over to the SDX interface, in System / Channels click on Add
Click on Add or OK and go to plug your two SDX management network cable into the two network port configure with LACP on your switch.
You might need to refresh the SDX webpage to get back on it. Then if you check in System / Channel you should be good !