I’ve been testing this more today. You definitely have to have DHCP working on some other network first. Then you can enable AP mode. Then you can login to the router again, there is an “Advanced” tab, where you can go to the OpenWRT configuration, change the IP to static, set it to whatever you want, and then save it. Its not elegant, but it works eventually. It would be nicer if AP mode just let you set it if the IP is set statically already.