Welcome and thanks for posting. If you have multiple PCs, you may want to look into GlassWire. Community members have suggested and used GlassWire, so feel free to ask them for tips on using it: https://www.glasswire.com
As for data monitoring apps for your mobile devices, there are plenty on their respective app stores. I'll defer to our community for their input on mobile data monitoring apps they've used.
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Very easily! In OSX click on Finder, select "GO", from menu select Utilities. Top of the list will be "Activity Monitor". Open it and select help to find out how to use it. You will see as much as you want. Showed me that Facebook is a data "HOG", as is the Hughesnet home page. After 27 days I have used 2.8 and 6.7 Gb of my 5/50 plan. Buy a Mac, and "Never go back"!
WRONG! I check any device I want, can graph, print, or what ever I want. I can check each application, which is why FB and the Hughesnet home page are not used. I've debugged the network for all ATM's in Canada. Which IBM wrote and could not debug in ten years! Also designed and set up aircraft maintenance for all airports in Canada. Not to mention designing, developing, and installing Owen's Corning's world wide corporate salaried payroll system. That is just for starters. The PC is a design that we said would not be secure back in 1980 when the first XT was released. Now in 2016 is has proven our theory correct. Just because Bill Gates worked at IBM developing IBM DOS, and left to write his own MS-DOS, did not make him anything but a "Pirate".
Is it only the more expensive Asus routers that can do this, or is it all of them? I'm not going to do this, I'm just curious. I already have a router that works fine, and I only use it when I use my laptop, which is about a third of the amount of time I use my desktop. I actually have two routers, but one is getting a bit buggy.
AMD FX-6100 | Samsung 250GB 840 EVO SSD | Western Digital Blue 500GB HDD | 16GB DDR3-1866 | EVGA Geforce GTX 550ti | Windows 10 Pro 64-bit