That’s true, GlassWire isn’t specifically made to check for an entire network outage but in some cases it can tell you if your network activity was lost.
One trick you could try is to use some kind of service or stream to another server that you know will constantly load throughout the day, then check for this connection on GlassWire’s graph. If the connection went down and there was no network activity for awhile perhaps your network was down. Maybe you could stream an Internet radio station or something like that.
This technique isn’t perfect though of course.