This support article guides you through using the SD card for the UI files: https://opensprinkler.freshdesk.com/support/solutions/articles/5000011026-using-the-sd-card-for-ui
The device is controlled using the network connection. With that said, you don’t have to buy a router. You can connect it directly to your computer using a cross-over cable. Also, the SD card is only needed for logging of data.