You need to know the IP address of your OpenGarage and just type in that IP address in a browser to access the homepage (and options page too). If you don’t know the IP address, you can find out on your router’s configuration page, or you can use a thin screwdriver to press the button on OpenGarage for 3 seconds and release, it uses audible sound to inform you the IP address, as described in the user manual:
https://github.com/OpenGarage/OpenGarage-Firmware/raw/master/docs/OGManual.pdf