How can I monitor data usage on my router?

[Geek-9pm]

This question is for everyone. 
Please relate whatever you know about this topic.
I have already done some searches and have become very sad. 
It seems that the big data business tells about data usage. So a number of savy IT guys did there own testing and found the company would charge them even when the modem was disconnected from the cable. 

[DaveLembke]

Other than Xfinity/Comcast raising my bill recently by $28.75 I haven't seen any data usage fees yet.

For monitoring how much data was being used with my Windows 7 systems I use to use a Network Monitor Gadget that showed how much data has been uploaded and downloaded. I then could keep an eye on usage of my computers which used the majority of the bandwidth.

More recently, I used a tool that allows Windows 7 gadgets to run under Windows 10. ( Even though Microsoft Killed off Gadgets due to security vulnerability.) http://win10gadgets.com/network-meter/

I have yet to find a router that tallies data usage.

Only method I can think of to tally data would be to setup 1 system as a gateway with ICS  ( Internet Connection Sharing ) where that computer has 2 network adapters in it and works between 2 routers. One network connection that connects to the internet to the first router for hardware level firewall security, and the other connection from the PC that connects to a second router with shared internet connection. That computer that works in between the router using a gadget to keep an eye on upload and download use. The gadget can be easily reset and start back at 0 for both stats whenever you want it to, such as monthly. You could then keep an eye on what your actual usage is including any data usage that the computer acting as a gateway uses for Windows updates etc.

Here is info on ICS ( Internet Connection Sharing ) for Windows 7. https://www.home-network-help.com/windows-7-internet-connection-sharing.html

I haven't used ICS since Windows XP but its pretty simple to setup.

Only drawback to this setup is that you have the additional electricity cost and a small amount of added latency. If you have a low powered computer like a laptop the electric bill cost of having a system running as an ICS Gateway wouldnt be that bad at around 45 watts average. A desktop computer can be between 45 watts and 125 watts depending on what processor you have running and whatever else it may be doing in addition to being idle and passing packets through it as a tally.

[Salmon Trout]

My (UK) Virgin Media "Super Hub" supplied router, a rebadged Netgear model, shows usage history since last power cycle: