How to Determine Data Use by Device

Monitoring data usage by device is generally a feature found in higher end routers. The TMO gateways have limited functionality.

You would need to invest in a router that has that specific capability attaching it via an ethernet cable to the TMO gateway. Configure the new router as a router (some do it automatically just by plugging it in to the TMO gateway) and then insure all of your wireless connections go through the new router by disabling wireless connections on the TMO gateway while being aware that this overall approach has some NAT ramifications discussed throughout this community.

There are some applications that can be run on various device to do this also but those utilities need to be installed on each device and not all devices are supported. Glasswire comes to mind but is only available for Windows and Android at this time (other smart devices aren't supported).

Overall monitoring through a router that supports data usage monitoring and ensuring that each device connects through that router is the best solution while providing some additional features (most importantly security features) that aren’t provided by the very basic TMO gateways.

Personally I prefer any of the Asus routers having AsusWRT with the more advanced Traffic Analyzer feature (not to be confused with the Traffic Monitor feature - i.e. make sure the spoecs explicitly state Traffic Analyzer because that's what you want to do). Other brands have similar features.

Otherwise it’s a process of elimination as noted above.