FMM003 Roaming Issue: Connection drops to Operator 0 after a few minutes with Three HK SIM in South Korea

Telematics Issue
1

Hello,

I am experiencing a persistent network dropping issue with a specific roaming SIM card on my FMM003 device. Below are the technical details and the troubleshooting steps I have already taken.

Device Information:

  • Model: FMM003

  • Firmware Version: 04.02.00.Rev.602 (Latest)

The Problem: I am using a Three HK (Hong Kong) Roaming SIM in South Korea. The device successfully connects to the network initially, but the connection drops completely after a few minutes.

  1. Initial State: After power-on, the device shows Operator Code: 45005 (SK Telecom) and GPRS: Activated. Data is successfully transmitted to my server (flespi) during this period.

  2. The Drop: After approximately 3–5 minutes of stable operation, the Operator Code suddenly drops to 0 and GPRS becomes Deactivated. The device fails to re-establish the connection on its own.

Troubleshooting performed:

  • Smartphone Cross-Check: I tested the same SIM card in a Samsung Galaxy phone. By disabling automatic network selection and manually selecting SKT (450 05), the data roaming works perfectly and stays stable. (Note: It is a data-only SIM, as voice calls are restricted by the provider).

  • APN Settings: The APN is correctly configured as mobile.three.com.hk. Since data is transmitted successfully for the first few minutes, I believe the APN settings are correct.

  • Comparative Test: I tested the FMM003 with another brand’s roaming SIM card using the same settings (Operator 450 05), and it works perfectly without any drops.

Questions:

  1. Have there been any reported compatibility issues between the FMM003 (or its cellular modem) and Three HK roaming profiles in South Korea?

  2. Since the connection is stable on a smartphone but drops on the FMM003, could this be an IMEI-based blocking issue by the roaming carrier, or a timeout issue during the GPRS handshake?

  3. Is there a specific configuration or AT Command to force the modem to stay registered on PLMN 45005 and prevent it from dropping to Operator 0 when a session fluctuates?

I would appreciate any insights or recommended configuration changes to stabilize this specific roaming SIM.

Thank you.

2

Hello,

Thank you for using out Teltonika Community Forum.

Thank you for the detail explanation. Based on the explanation you provided, you are using 3 Hongkong Local SIM with roaming capability enabled to South Korea. You have tested and verify that the SIM is able to roam in South Korea by inserting the SIM to mobile phone.

The issue you are facing now the connection established but drops afterwards.

This situation seems to be related to Data Acquisition setting. Have you check the data acquisition setting and is it the updated data to server following the send period setting for Roaming and Unknown? This might be the issue where it is updating once then drop as it have send period time setting. Further detail configuration manual available in our wiki for your reference : FMM003 Data acquisition settings - Teltonika Telematics Wiki

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You may also have a look into Records Settings under GPRS tab. This is to control the open link timeout between device and server. You may increase the timeout setting to prevent the link being close prematurely. Further detail information about this configuration available in out wiki page for your reference : FMM003 GPRS settings - Teltonika Telematics Wiki

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Regards,

Muaz.Z