Unfortunately, there is no driver available for Windows 8.1. According to the developers, you had the possibility of installing this modem on Windows 7 with a custom driver update. The link they provided for Windows 7 is discontinued which means it's impossible to download the file again. You can still get it through Google Search from 3rd party websites, but make sure you have a working antivirus installed on your computer to prevent malware from spreading in case you download something wrong.
Google Search: https://goo.gl/zmdX0s
Note: It's confirmed that Windows 7 drivers are actually working on Windows 8.1.