Another way is to use /etc/rc.local
I use that for Atlas service that does not start correctly upon reboot. It can be done via editing the file above or via http://192.168.1.1/cgi-bin/luci/admin/system/startup > Local Startup tab.
I have it like:
Update 2021-02-22: This “solution” does not work. See my next post for a working solution.
Thanks for the replies!
I first tried adding a new file root to /etc/crontabs containing @reboot service firewall restart as this appears to be the more “modern” way. Unfortunately that did not work (and I did not have the time for further investigation).