Mimi1
In addition to the Glasswire mentioned by maratsade, many devices have built in data usage monitoring apps/programs. This is especially important for those devices that can't utilize Glasswire.
Also, in case it may help, you can see your currently connected and disconnected devices on this modem page. The "disconnected" devices are those that were connected, but are no longer, since the last time the modem was plugged in/powered. When the modem is unplugged or loses power, the logs are erased. This information can be helpful when there are devices connected, and still using data, that may have been forgotten about.