Hello,
There is no known issue regarding your question, and I have not managed to replicate the issue.
Some traffic is to be expected, as the device continuously performs data connection health checks by sending ping requests to a specified host. In this case, you could reduce the frequency.
I haven't performed long term tests to evaluate the data consumption, but 300-500Mb/day seems way too much.
For this I would like you to, first, as suggested in one of your previous threads, reset the router to factory defaults.
Also, do you have any of the remote access options enabled, such as HTTP, SSH?
Next, you could monitor traffic on the mobile interface with TCPdump, to see, where the traffic is going to and to make sure sure the router is not acting as suggested.
You would need to acces the device via SSH, download tcpdump with the following commands:
opkg update and opkg install tcpdump.
To monitor mobile interface you would need to execute the following command:
tcpdump -i wwan0 -w /tmp/capture.pcap
The using WinSCP, trace file could be extracted to analyze with wireshark.
Best regards,