I know this is an older issue, but it's still the first one that showed up in google when I was searching, so I'm gonna post my solution here. The issue is that the start time and end time are misleading. Start time refers to when the guest network will shut off, and end time refers to when the network will start back up again.

So if you want the network to be available from 6pm to 10pm, you would need to add 12am to 6pm, and 10pm to 12am to the schedule (I don't think it lets you put 10pm to 6pm the next day).

I had it backwards the first couple times I tried setting it, so the network would automatically switch to disabled whenever I hit save, as is described in the original post.