you can get this to work (tested on unleashed on a r610 , latest unlsh version as of may 2022), but i cant confirm that it will be saved / persist across reboots (but it *should*). Unfortunately, as usual, Ruckus standalone firmware has the most features, so lets jump back to standalone FW syntax to get this to work on unleashed:
1st- SSH into the AP you want to change its 2nd ETH port's untagged vlan to. (you have to do this on each and every AP you wish to change)
(if its the unleashed master after SSH prompt, you will 1st have to do, enable then ap-mode)
then run: get eth (to confirm you are about to switch the CORRECT ETH PORT, as if you change the wrong eth port you can loose your connection to the AP and possibly be locked out fully) - as a general rule: be sure you are not changing the eth port that has POE marked on it (as this is MOST LIKELY the "uplink" port your ssh session is running over).
set interface eth1 type access untag 702
(where 702 is the VLAN you want as untagged on that 2nd eth port).
to see other options (like setting it back to a vlan trunk port), issue command: set interface (press return to see the help, or i think set interface ? will give you the help as well). the help screen is very good on ruckus CLI btw!.
also this feature should 100% be added to unleashed web GUI ASAP. if you see this link below (back from 2020!! this functionality is only available on 2x unleashed AP models, which is absurd). This inability to change (or even DISABLE) the 2nd eth port of unleashed APs actually can be a pretty big SECURITY issue in most environments / configurations.
(its GREAT that ruckus gives us so much functionality on the Standalone Firmware, but much of that still needs to be brought over to Unleashed. While it does have its bugs, Unleashed really is a GREAT platform)