How to configure OpenWRT as access point
You equipped an old router with the reliable OpenWRT to save the planet a bit? Very praiseworthy. Often these are simply used as additional access points and we show you here how to configure OpenWRT accordingly.
First you connect the appropriate router to your Mac via cable and call the web interface under 192.168.1.1. Then go to Network > Interfaces and select the "LAN" interface (click on the blue "Edit").
Then assign a fixed Internet address to the access point. If your main router is 192.168.1.1 and there are two switches
in the network with 192.168.1.2 and 192.168.1.3, then we assign 192.168.1.4 (or as you like, it just has to be in the same subnet, 192.168.1.x). As gateway you enter the address of the main router, also for the DNS server, because the main router should be responsible for the address assignment.
In addition you switch on the option "Ignore Interface" at the bottom of the settings under DHCP Server.
Under IPv6 Setting you set everything to "disabled". Then press "Save and Apply".
Then change via menu bar to System > Startup. Since we only use the router as a translator between cable LAN and WLAN, we can simply turn off the firewall and the two services that normally take care of DHCP. Set the three services to be seen to "disabled" and press "Save and Apply" again.
Now you can restart the router and it should be accessible under the address you just set (here in the example 192.168.1.4). If it is not, you probably forgot to press the blue save button - so do the whole thing again. 
Plug the cable coming from the main router into one of the switch's four ports (not the WAN port). Done!