Hello,
I have OpenWrt 21.02.1 r16325-88151b8303.
After booting, the installed mosquitto-broker does not run. I always have to start it manually via LUCI or command line. The service/initscript is enabled.
How can I find out what the problem is? Maybe the network is not ready during the start of mosquitto?
Might be race condition, try adding a restart to the local startup script.
As workaround a good idea. 
I had mosquitto running successfully on a GL.iNet GL-X750 (NOR). Unfortunately I broke that tenet - if it ain't broke don't fix it!! Upgrading to the latest GL.iNet v 3.212 firmware for this device the mosquitto auto boot at start is no longer working. No problems with starting mosquitto from the command line but
/etc/init.d/mosquitto enable
/etc/init.d/mosquitto restart
has no effect anymore.
I've uninstalled and reinstalled mosquitto-ssl, recreated the passwd file and checked the config file.
Would anyone know how I'd check that the installed /etc/init.d/mosquitto proc script is configured correctly?
I should add that LuCI is showing OpenWrt 19.07.8 r11364-ef56c85848 as the current version.
If you're using gl.inets firmware, you really should ask them.