10Kb per hour received and 18Kb per hour transmitted while idle

I am running a Teltonika RUT240 in my RV to monitor battery usage, power and temperatures. These measurements are collected by a Nodemcu ESP8266 device and transmitted to my MQTT server at home.
Because my RV is on the move and frequently travels outside my home-country, i try to keep the 4G data costs at a minimum. I have determined i cannot get received (rx) data usage below 10 kilobytes per hour and transmitted (tx) data usage below 18 kilobytes per hour, even if the RUT240 is idle. This data usage still occurs when:

  • i have firewall rules set which block all traffic from lan to wan (including the mobi interface)
  • RMS connection disabled
  • no VPN
  • MQTT server and client disabled (but i did install the mqtt package)
  • the only thing thats happening is that ‘Data to Server’ uploads the tx and rx values to my mqtt server every half hour. But that should incur bytes of usage, not kilobytes.
  • i’m not sure if syncing to a time server is enabled.

I would expect a data usage of just a few hundred bytes for the 30-minute ‘bytes tx/rx’ values to upload. How can i investigate where this data usage is coming from? I had traffic logging enabled, but that showed 0 bytes every time because no connections were made or closed (on the LAN), affirming my assumption the data usage is caused by something on the RUT240 itself.

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