Thank you Peter for the swift reply.
That's what I was trying to do but somewhere along the way I failed. The steps I took are all posted in the screenshots above. I fail to turn the LAN1 port on the secondary OpenWrt router into a WAN port, well into a working WAN port.
I need someone to show me how to set it up.
Is it okay to keep the network LAN side at the static 192.168.1.1 IP address while designating a static IP address to the WAN side which is the same subnet like the primary router, e.g. 192.168.100.2?
The primary router's IP is 192.168.100.1 (also posted above in the screenshot).
Also I recon that the OpenWrt router should have the DHCP switched on for the devices or do each of the devices need a static IP address?
When I do what I did in the screenshots above I got no internet on the OpenWrt router.
So somewhere along the way. I screwed up.
I wish someone could show me what I am doing wrong.
Thank you for taking time to reply Peter.