Sorry for the newbie question, but could someone point me how best to setup our home network. I have an OpenWRT installed router (with firewall/DHCP server) which is wired to a WiFi access point. I then want to add another WiFi access point over a wireless connection to help extend the network to the other side of the house. Should this be a mesh network? and if so should the router be part of the Mesh configuration?
Mesh networks (802.11s) are wireless backhauls, they function almost like "wireless ethernet cables", if they existed.
No phones and laptops can connect to a mesh network, it's like you trying to connect to a ethernet cable, you can't. That said, it's obvious your router can't be part of the mesh network.
Now, I have a feeling a mesh network is not what you want... It seems to me you want your AP2 to repeat the wifi signal of your AP1, right?
Maybe consider using 2.4GHz inside your house, 5GHz signal doesn't like big walls, if you want your wifi to reach to the other side of your house, enabling a 2.4GHz wifi with the same SSID as the wifi you use for 5GHz is an easy solution. Devices will seamlessly roam from 5GHz to 2.4GHz when signal gets too low.
See if you can avoid this.
Possible options, in addition to UTP cable, are the use of power lines and coaxial cabling.
Marketing of proprietary "mesh wifi" home network solutions has led to the term "mesh" being misunderstood. In OpenWrt, a mesh is strictly a backhaul technique. Enhanced roaming between APs is a separate issue entirely.
If you can't make a wired backhaul connection to AP2 (which as others said, is highly preferable) you could use WDS between the two wireless routers. This is simpler than setting up a mesh. A mesh with only two nodes doesn't do anything that a WDS AP-STA connection can't.