Debugging some issues with OpenWRT 19.07 (firewall?)


I have a NBG6616 running with 19.07.
From time to time, and more and more often (more or less once a week now), my iot device cannot discuss each other. They are using 2.4GHz network and it seems that only those using this network are affected. Or say it diffrently, all our smarpthones / laptop etc are working normal.
So far, I was rebooting the router in order for everything to be back to normal. But I found it was a pity to use such brutal solution. Trying different stuff like restarting Wifi 2.4Ghz network without success, I found out that restarting firewall service was enough to make everything back to normal.
How can I go deeper in debugging to understand what's going on?
Or may-be should I schedule a router/firewall reboot every night?