The SOTG will not allow any traffic to go out of that USB modem, until reset/value changed if the data quota is set. If you could verify that the SOTG is on current firmware: http://download.pepwave.com/firmware/sotg/fw_010022_build_1219.bin
If the unit is on the above firmware build and is still passing traffic through the USB Modem after it reaches the Data Quota limit please open a support ticket on the following link. This will allow a technical member to take a closer look: http://cs.peplink.com/contact/support/
I will also, ping the advanced team regarding the warning limit set to see if there is a reporting service such as sending a email/text once it reaches the warning limit that is defined.