Strange thing. But I assume this should proof that hardware video acceleration is working which was not the case with the initial Android 7 version on FP2.
You could boot into dic:safemode and keep your network active and play back videos using the standard app (not sure what Android 7 has to offer here). Not sure which file browser is on Android 7, Lollipop had Amaze installed.
So you could test if the stock OS causes the heavy network load. Also enable/disable WiFi to maybe distinguish if it’s always the same or only on WiFi or only on mobile connection.
If things are good then I’m afraid there is more work for you to find the specific hungry app.
A simple way prior to uninstalling apps is to “force stop” one suspect after another.
I do so with specific apps which actually do not really “quit” after closing them, but rather only kill their gui but stay active in the background (Osmand navigation, MXPlayer, although it has a ‘quit’ option).
Often users rely on a wrong assumption here.