The Unified firmware does require the Internet to load the UI files when accessing the control directly by IP address. However, the application does not require the Internet to communicate with the controller.
To troubleshoot, can you you ensure the correct IP address for the Pi by running
ifconfig. Once you have the IP, you can try to access the firmware on port 8080 (the default is port 8080) by using the following URL: http://192.168.1.88:8080/su. This is the script change URL which does not require any Internet connection render. If this works, then the app should work as well using the same IP and port.
If you wish, you can also host the UI files on the Raspberry Pi by using Apache or any other web host. We have instructions available here: https://opensprinkler.freshdesk.com/support/solutions/articles/5000164006-using-a-different-server-for-ui-assets
If this continues to not work, you may be best served by sticking with the Pyton program although the Unified program should work in this setting.