I think i sorta bricked my ax3600 by checking "clear all configurations" when i was downgrading the firmware. I think that option cleared all some configuration files such as "/etc/config/wireless" in the firmware.
Update:
Managed to "recover" by using the /etc/config/wireless file in my ax1800 and put it back manually to ax3600. Downside is factory reset does not have the file so if i do reset, i need to manually create the file again. Also the LED on the antenna is no longer working.
Also after changing the region back to china, i managed to get the mesh working between ax3600 and ax1800. So before you change the region from China to whatever, make sure you have the mesh up between the routers. One thing to note is the ax1800 doesn't have an international firmware as of now. So for compatibility, CN region probably work best for both.
Update:
I have removed the crash partition and it solved the issue. I factory reset the router and everything is back to normal. Remember to remove write access bdata partition if not you get will your wifi bricked!