@Daniel-nio
Hey and welcome
The non displaying of the stats in DEVICES is a bug that appears in v5 of SDN, its annoying so hopefully they will fix it soon
In answer to your question, the 338 / 51 you are seeing is the volume of "internet" you are using and this is pretty accurate I find.
However don't rely on the Switches / APs data as its blended with local traffic, ill try to explain. Not everything in your network goes over the internet, say a print job for example. Laptop 1 sends a 10mb print, this goes to AP1 which it is connected to, then to switch 1, then to AP2 then to printer. Therefore AP1 AP2 and the switch have all recorded 10mb of traffic, nothing went online so the router records ZERO traffic. Add on to that Air Drop, Roaming, Controller Traffic, Updates, Local DNS, etc etc etc none of which need to go "online" so therefore the stats on the Switches / APs are massively more than the router.
This is perfectly normal behaviour, I would be much more concerned if that wasn't the case! Local "LAN" traffic over APs and switches will always be massively different to the WAN "internet" link
Just for reference, here is my stats for the last 7 days. Its the same for me
Router (ER605)
Switch (2008P)
AP2
Hopefully that helps!