Ahoy ahoy friends.
I'm having some trouble with my DynDNS OpenWrt setup, and didn't fix it during almost 2 years, so i'm requesting help of some OpenWrt veterans 
I got one cable router and one DSL router, both connected to my OpenWrt device through mwan3.
So in both cases, OpenWrt runs behind NAT.
mwan3 is configured to use only WAN interface and WAN6 as long as they are available. In case of failover it should only use WANB and WANB6. Unfortunately DynDNS does work "sometimes" but i don't know how to properly configure it. Sometimes it works, most of the times not. Also i want it to start automatically on boot, and keep updating every X minutes without having to manually stop and restart the service.
I hope someone could help 
All confidential information has been omitted.
The target hostname of the IP checker URL is properly configured, to show the IP address of the current active gateway.
Also i don't know about the event interface. Which interface do i have to choose? I have absolutely no idea!
That's what the logs say:
204051 ERROR : Can not detect local IP using 'network_get_ipaddr wan' - Error: '1' - TERMINATE
204051 ERROR : Can not detect local IP using 'network_get_ipaddr6 lan' - Error: '1' - TERMINATE