Thank you for your support. I have tried to config as your description, the connection seems fine for about 4 days, after that, the timestamp is wrongly updated. Only after I unplugged and plugged the power supply, 8 hours later, the timestamps is updated correctly.
When I took a look at the Network log, I notice that the Mobile data will be refreshed at UTC 2:47AM everyday. However, until Jan 9, the RUT mistakenly updated wrong Timestamp. After that, the data are still sent but when the timestamps are incorrect, these package are rejected.
I noticed that in the System log, there is an event LAN plugged in appear many times, at the same time as the incorrect timestamp updated. Thus, I assumed this may a problem with the system.
Can you please help me checking on this? Thank you so much.