@Liftboy 

as has been suggested earlier , try configuring a 'keep alive' to maintain the routers NTA connections

Seems the Fritzbox does have that facility

If you enable port forwarding in the FRITZ!Box, the FRITZ!Box sends IP packets at regular intervals and the router's port forwarding automatically remains enabled:

Click "Telephony" in the FRITZ!Box user interface.

Click "Telephone Numbers" in the "Telephony" menu.

Click on the "Line Settings" tab.

Click "Changing the Settings" in the "Telephony connection" section.

Enable the option "Keep port sharing of the internet router enabled for telephony".

Select a suitable time interval. Select "30 sec." if you do not know how long temporary port forwarding rules will be maintained.

Click "Apply" to save the settings.

Thank you very much for your efforts! 

I have set this up on the Fritzbox and put the DSL back on the Omada router. I have set the IP of the Fritzbox as DMZ in Transmissions / NAT / Port Forwarding. I also set SIP ALG sometimes on and sometimes off.  Then I entered the DNS servers of the provider in the Fritzbox 62.109.121.1 / 62.109.121.2 Unfortunately no success.

I still have to work today and realise that the lack of time doesn't help. First of all, I'll reset the DSL to the Fritzbox and let it handle the telephony connection. On the 2nd day of Christmas I will try again with time and patience.

THX ;)

Frank

Hello!

I now know that my provider's VoIP connection must also run via the DSL. I have set up Port forwarding for port 5060 and selected the interface of the DSL and the destination IP of the Frizbox. DMZ is ticked.

But ;) How can I define that the Fritzbox with its IP (as IP client) is always served via the WAN connection of the DSL?

I have made a static route DSL WAN port to the Fritrzbox IP ...

Won't work till now ;)

Regards and thanks!

Frank

Hello!

I seem to have solved my problem with telephony.

Two Internet connections, 1x fibre optic and 1x DSL. The DSL line also includes VoIP telephony via the provider.

Structure:
* DSL - Vigor modem via PPoE
* Fibre optic
-> Router TP-Link ER707-M2
-> switch
-> Fritzbox as IP client for DECT telephony
-> various access points

Problem: No telephony possible (Fritzbox protocol = DNS error)

Measures I have taken.

Fritzbox:
- Configured as IP client
- changed to static IP, DNS pure
- Changed the DNS server of the fritzbox to that of o2 (ISP) (62.109.121.1 ; 62.109.121.2)
- "Keep port forwarding of the internet router active for telephony" set to 30sec
- enter "sip.alice-voip. de" as proxy+registrar

TP-Link Omada Router ER707
- SIP ALG on
- Port Forwarding (Transmission/NAT) Limited IP source Fritzbox, Destination IP Fritzbox via DSL WAN interface
- Static route IP Fritzbox via interface LAN and WAN

One of the last two points did the trick.

As I don't have a full technical understanding, it took a while. The last points won't necessarily be logical either - but it works.
 

Which will have been very important. The Fritzbox was in my network via dhcp (static lease) in my network the dhcp assigns the dns servers, which seem to be preferred by the fritzbox. So fritzbox switched to static ip, dns in!
 

Now I have a fibre optic connection with 250mbit down and 50 mbit up, like a DSL connection with 100 down and 40 up.
 

In the router I have selected load balancing with a weighting of 3:1 for the fibre optic connection, as well as Application Optimised Routing.
In my understanding - in terms of speed, there will probably be no advantage in combining the two technologies.
 

As soon as I activate Link Backup on the router, the fun with telephony is over. My assumption is that the DSL is no longer active and the telephony MUST run via the DSL at the provider.

So much for the feedback. Many thanks to you for your support!

Frank