The IP address is NOT the same with WiFi vs. with wired Ethernet module. Please, always press button B1 to check the IP address assigned to it.
“DHCP disabled as the site will not always have internet access” — I do not understand this — your router’s DHCP feature has nothing to do with Internet access.
“internet connection is required to access the controller because of Java” — no, this is NOT true. You can use the OpenSprinkler mobile app which does not require Internet. It only requires Internet if you use a web browser to directly access the controller. Also, it does not use Java, it uses Javascript.