Thanks for your answer. I would be grateful if you could clarify one small bit for me as well please.
I read somewhere that a router is a device that sits on 2 networks in this case OpenWrt sits on 192.168.2.0 the WAN and 192.168.1.0 the LAN and has 2 IP addresses 192.168.2.2 and 192.168.1.1.
According to Wikipedia:
A default gateway is the node in a computer network using the internet protocol suite that serves as the forwarding host (router) to other networks when no other route specification matches the destination IP address of a packet.
If we think of the OpenWrt router from the LAN perspective then isn't the WAN IP (192.168.2.2) the forwarding host to other networks? Or because it is the same host it doesn't "count"?
On the other hand if we think about it from the WAN perspective then it makes sense that 192.168.2.1 (modem/router's LAN IP address) is the default gateway.
Also should I then fill in the greyed out bit explicitly?