@DiliMe after half a year.... ;-) I did something similar; maybe this helps? https://www.snbforums.com/threads/port-isolation-on-rt-ax88u-pro-router-w-merlin-w-tagged-internet-isp.89677/post-902942
Paul
I finally managed to get a working solution; although technically it's not entirely clear to me yet, at least I got it working.
First of all I configured a guestnet in the GUI (2.4GHz only, no intranet access)
Then I got following bridges (configured by Merlin):
# brctl show
bridge name...
As a newby I have been trying to configure my RT-AX88U-Pro router to isolate a camera system connected to LAN poort 2 from the rest on the LAN, so basically to setup a guestnet which only has access to internet.
Funny enough with Asus stock firmware I can setup a VLAN and configure LAN poort 2...