What I do, and what works on 18.06.04:
Having custom built image, from sources, no LuCI, doing several mods to system files (i.e. /etc/config/wireless,network ) during initial run of /etc/rc.local after flash, these changes are kept during reboot, which is triggered from rc.local itself, after applying changes.
So, the process is:
On very first boot after flash, rc.local does some mods to various system files, and triggers reboot at end of script. Mods are kept.
However, using image, based on most recent files, these mods are not kept any more during reboot.