I'm a licensed WiFi Guard user and quite happy with it. But I've just installed a TP-Link RE650 Wi-Fi Extender and as a result my trusted devices are now reported as "Unknown Device" twice every time. I noticed that the Extender uses variable MAC addresses replacing the first 6 characters with its own. For example, if RE650's MAC is AB:FE:4D:EE:FF:3C, then devices connected to it are reported as AB:FE:4D:last 6 chars of the device.
I thought that I could get around this problem using WiFi Guard's Partial Match feature. I added a partial match as AB:FE:3C and a comment, but it does not work as the devices are still reported as Unknown.
It seems that the partial match string should be AB:FE:4D (instead of AB:FE:3C that you entered).
Additionally, once a device was found to be "unknown", it will keep appearing that way until the user takes action. This is to make sure that nothing sneaks in without the user's knowledge.
To resolve your situation, add the partial match, then open All Devices and "Forget" those unknown devices. After that, do a new scan, during which any MAC addresses beginning with AB:FE:4D should be automatically whitelisted.