I have a Nighthawk RAX54 modem and an older Surfboard SB6190 router.

Just a tiny technical note:

• Surfboard SB6190 is the cable modem, and
• Nighthawk RAX54 is the WiFi router.

It is indeed frustrating that Netgear does not provide any method to report network usage by device, which would immediately identify "what's going on."  This is the top request on Netgear's Idea Exchange since 2018:

https://community.netgear.com/t5/Idea-Exchange-For-Home/idb-p/idea-exchange-for-home 

The Traffic Meter is a poor substitute.

plemans has a good point that a sudden increase in data usage can be an indication of something running wild. In other words, "nothing changed" could mean "as far as I am aware or intended."  Finding the device can be a nightmare.  

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Yoda74 wrote: 

2) My Netgear Nighthawk RAX54 router shows half the usage in its 'Traffic Meter' report of what Xfinity started to show for September and now October... 

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No matter which devices are using the network, the quantity of up/down data reported by the router should match the Xfinity report.  Do they both start counting on the 1st of the month?  Does Xfinity provide a convenient way to compare usage with the Traffic Meter report?  (such as reporting "Yesterday")

I was just looking at the Traffic Meter report on my Netgear Orbi and find the numbers hard to accept.  It says last month I uploaded just under 1GB/day and this month I am uploading 12GB per day!  For the entire month of September, I downloaded 242GB, and through under 10 days of October it claims I have downloaded 980GB!

Alas, I cannot compare Netgear's Traffic Meter against Spectrum because Spectrum does not provide usage information.