If your primary internet usage device is a Windows based computer, try Glasswire. It will tell you what is using your data on that computer. The free version is fine. https://www.glasswire.com/
It will only tell you what data is used after it is installed. It can't look back.
If a Mac, there should be an app called Activity Monitor built into the Mac OS. It monitors data, though I don't know if it breaks it down like Glasswire does. iPhones should also have an Activity Monitor.
If a Windows based PC, all sorts of things can use data, even all of a sudden. Syncing, updates from more than one place, an update that is stuck and is trying over and over again to download, the cloud (OneDrive).
If you have a satellite TV receiver connected that can use data, too, even when not watching something on demand and even when it's "off" (it's not really off, but in standby).
There is a problem with exede. It is blocking sites when in data restriction. It also seems like it is counting overnight data against your quota. You do not need to buy another router or monitoring software. If they don't fix it, switch to Hughesnet... I don't want to, but I'm not going to accept this!