I’ve noticed that Plex never shows me “real” ads when I use the Plex app on my computer to watch TV shows from Plex. It only shows me ads for other Plex shows or movies and very rarely I’ll see an Ad Council ad. I do get actual ads if I watch on iOS or on my Apple TV or via my web browser. It seems to show the same length of Plex promos that I would get if they were actual advertisements. I’m just brining this to your attention because it seems like you’re missing out on a ton of ad revenue.
I have nothing that would block ads on a network level. I do get actual ads on this same network using the iOS player, Plex for Apple TV or from Safari. I’ve tried it from two different Macs and both do the same thing when using the Plex player for Mac.