One possible source of data consumption is the wireless access point. By default the RAK7268C is not password protected, and devices can connect to it and access the internet. I recommend you either disable the wireless network or you configure a password.
Another source of data usage is the cellular interface tracking settings. By default, it has a ping interval of 5 seconds. A ping packet is 74 bytes, a simple calculation gives you about 37 MB per month. I recommend augmenting the ping interval.