Hi,  @SFA1492 

Thank you for posting on our Business Forum.

Regarding the issue you raised, we would like to collect additional details to help resolve your problem:

1.Could you please confirm whether you manage your router via the IP web interface, and whether you have attempted to complete registration via Controller Mode?
2.Please kindly provide your full network topology for our reference.
3.When attempting registration on the ER7406, have you reached out to your SIP provider to verify whether they have received your registration request?
4.Have you attempted troubleshooting by configuring port forwarding or enabling DMZ?
5.You mentioned that this problem started occurring after you updated the COMtrexx firmware to version 2.6. Could you please confirm whether you encountered any ER7406 registration issues before the firmware upgrade?

  @Nathan-TP 

Thank you for your reply.

Please find my answers below:

1. The ER7406 is managed via the Omada Controller (OC200). I have not tested standalone router mode.
2. Network topology:

The setup with the ER7406 uses an ALLNET ALL-BM200VDSL2V VDSL modem in bridge mode:

Telekom VDSL connection
|
ALLNET ALL-BM200VDSL2V VDSL modem
|
ER7406 v1.0 (FW 1.3.2)
|
Auerswald COMtrexx PBX (FW 2.6)

During the test with the FRITZ!Box 7590, the ALLNET modem was not used because the FRITZ!Box established the DSL connection directly.

The COMtrexx is connected directly to one of the LAN ports of the ER7406. It performs SIP registrations to Telekom Germany and Peoplefone.

3. I have not yet contacted Telekom regarding the issue. However, the PBX reports "401 Unauthorized" for the affected Telekom accounts after they lose registration.
4. I have not tested port forwarding or DMZ because SIP registrations are initiated from the PBX towards the provider and should not require inbound port forwarding. Nevertheless, I can perform these tests if you believe they may provide useful diagnostic information.
5. Unfortunately, I cannot say with certainty whether the issue existed before upgrading the COMtrexx to firmware 2.6. The problem became noticeable only after the upgrade. However, after replacing the ER7406 with a FRITZ!Box 7590 while keeping the same PBX, SIP accounts and internet connection, all registrations have remained stable for several days without any failures.

Because of this result, I am trying to determine whether there could be an interaction between the ER7406 NAT/UDP session handling and the SIP registration behavior introduced or changed in COMtrexx firmware 2.6.

If required, I can also provide screenshots, packet captures, or additional diagnostic information.

Hi,@SFA1492 
Thank you for providing your information.

Our team would like to confirm a few additional details with you:

1.What was the reason you disabled SIP ALG on the ER7406? Have you had the chance to conduct a test with SIP ALG re-enabled?
2.Was SIP ALG also disabled on the FRITZ!Box 7590 during your earlier stability test?
3.We would like to ask you to perform a packet capture for further troubleshooting: please set up long-term mirrored packet capture on an idle computer, and configure it to automatically split and save capture files either at fixed time intervals or once the capture size reaches 50MB. If you are not comfortable sharing the capture files publicly, you may send them to me via private message.
Thank you for your cooperation.

  @Nathan-TP 

Thank you for your reply.

1. SIP ALG was intentionally disabled because Auerswald explicitly recommends disabling SIP ALG when operating a COMtrexx PBX behind a router.

In addition, I have encountered SIP-related issues with another TP-Link router (ER7212PC) in a customer installation. With SIP ALG enabled, outgoing calls frequently failed. Disabling SIP ALG resolved the issue immediately. Based on both the vendor recommendation and my practical experience, I disabled SIP ALG on the ER7406 as well.

Therefore, I have not yet performed a dedicated long-term test with SIP ALG enabled.

2. The FRITZ!Box 7590 does not provide a configurable SIP ALG setting. Therefore, I cannot determine whether any SIP processing mechanism was active during the stability test.
3. I am willing to perform packet captures for further analysis. Since the issue may take several hours or even days to occur, could you please clarify the preferred capture point and traffic direction?

Would you like to see:

• traffic between the COMtrexx PBX and the ER7406 (LAN side),
• traffic between the ER7406 and the Telekom SIP servers (WAN side),
• or both?

Additionally, are there any specific SIP hosts, ports, or capture filters you would recommend?

Thank you for your support.

Hi,  @SFA1492 
Thank you very much for your patience in providing all the information to us.

Below please find our responses to your questions regarding Packet Capturing:

1.We recommend capturing traffic for both connection (LAN +WAN )segments to help us accurately identify where the issue lies.
2.It is not required to configure specific SIP hosts, ports or capture filters for the capturing process.
3.When conducting the packet capture, please check the IP of the problematic device and the approximate time when the issue occurred. 
4.Could you please also check your local network to see if any other clients encounter similar issues such as network instability or lag? If there are any other affected devices, please provide their IP addresses accordingly.

Once you have completed the packet capture, please send me a PM with the following information:
1.The packet capture files.
2.The WAN IP address of your device
3.The IP address of the problematic device and the approximate time when the issue occurred. 
Thank you very much for your kind cooperation.

  @Nathan-TP 

Thank you for your detailed reply and for the troubleshooting suggestions.

I am still investigating the issue and appreciate your willingness to assist.

As the problem may take several hours or even days to occur, I will need some time to determine the most practical way to perform the requested packet captures.

In parallel, I am planning to test an alternative router platform in the same environment to gather additional information and further narrow down the source of the issue.

I will continue to follow up on this case and will provide an update once I have collected additional results.

Thank you for your patience and support.

  @Nathan-TP 

I would like to provide an update on my testing.

In order to further isolate the cause of the issue, I temporarily replaced the ER7406 with a UniFi Cloud Gateway Ultra. The network topology remained essentially unchanged, including the same VDSL modem, internet connection, PBX, and SIP accounts.

After approximately two hours, the same issue reappeared:

• Three of the four Telekom Germany SIP accounts lost their registration.
• The PBX again reported "401 Unauthorized" for the affected accounts.
• The Peoplefone SIP trunk remained registered and fully operational.
• SIP ALG was disabled on the UniFi gateway as well.

This means that I have now observed the same behavior with both:

• TP-Link ER7406
• UniFi Cloud Gateway Ultra

The only router that has remained stable so far is a FRITZ!Box 7590, which I tested previously.

Based on these results, I am no longer convinced that the ER7406 itself is the root cause of the problem. At the moment, the issue appears to be related either to the interaction between the Auerswald COMtrexx firmware 2.6 and Telekom SIP services, or to a behavior that is handled differently by the FRITZ!Box compared to more traditional business routers.

To ensure reliable telephone availability, I will temporarily return to the FRITZ!Box while continuing to investigate the issue together with Auerswald.

I will provide further updates if new findings become available.

Hi,  @SFA1492 

Thank you very much for providing the relevant information.

Once you obtain the corresponding outcome after contacting Auerswald, you are more than welcome to share the result here. This will serve as a valuable reference for other users experiencing similar issues. Thank you!