"But some how, I got a notice that I only have %20 data left...... "
How did you get this notice?
It's because your account is showing the data you had remaining when you unplugged the modem. Your monthly data allotment can't reset if the modem is not connected and plugged in. As well, it's impossible for any data to be used while the modem is unplugged.
If your modem is connected and plugged back in, the data will reset.