Fritz!Box 7490 as VoIP Gateway in DHCP Client Mode on a LEDE based Router

Hi. So, I recently realized one of my projects and degraded my Fritz!Box 7490 to a VoIP Gateway by using it behind my TP-Link TD-W8980 with LEDE 17.01 installed on it. At first I thought that all is running fine, but I just did not realize that after a while no call comes through anymore. All my numbers are marked as synched, but no signal comes from the Fritz!Box, no beep, nothing. Just silence when calling myself from outside. When I reset the Fritz!Box all works for about 5-10 minutes and then it is silent again. Any ideas from someone who did the same maybe? Anything I have to add?
I googled of course and found this one having the exactly same problem (WARNING German) but I don't understand what it is good for at all and why it should be needed. Any help would be fine. If you need any logs, I can create em tomorrow when I am back home. Just tell me what you need.

Check and follow the first suggestion. I used to have that exact same setup for a short time, even down to the 7490 model, and I remember that exact problem and that the solution works.

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Thanks for the information. That was one of the steps I had to do. The other one was... factory reset the Fritz!Box to fix a somehow completely f***ed up config which ate my phonebook, DECT settings and more.

The Fritzbox is not bad, but also not 100% stable for VoIP. I remember that after one or two weeks of uptime it would stop putting calls through from inside and had to be rebooted. I can't put my finger on it, but it feels like it was not completely happy about being degraded to a VoIP gateway and not doing the routing itself.

But the Fritzbox was never intended to be more than a stopgap. In our shared office, we soon replaced it with a Gigaset 510 IP Pro base (for individual use, an even more inexpensive non-"Pro" Gigaset IP base would do fine, but we need more than two concurrent lines), and it purrs like a kitten, we are extremely pleased with it.

Would not really be a solution here. 2x Fritz!Phone C4 and C5 and Fritz!DECT everywhere. So we will see. Maybe it will get it's forwardings back if it does not like to work fine. Btw, Beta FW 6.86 completely b0rked VoIP in DHCP Client mode. 6.83 ftw.

I had similar issues with my 7390 that i downgraded into a voip gateway since the dsl chip is just garbage.
The only thing that helped me was to make a full reset, and redo every config manualy. Just dont upload a backup!
I guess that after a few dirty upgrades, some deeper configs must get messed up.

Tipp: deactivate ipv6 support. It doesnt work great either for voip on any Fritzbox. And since in client mode the Fritzbox have no firewall at all, its a a security hazard too. Ipv4 is still nated with your LEDE Router.

Sadly, there is no alternativ product for small customer (at least 5 concurrent lines) that dont cost you an arm and a leg. And well dont come up with asterisk, its a nightmare in matter of configuration.

PS: Even if you deactivate ipv6 for the box itself, clients on wlan and Lan on your Fritzbox should get an ipv6 directly from your LEDE Router .

A BNIB Gigaset N510 IP Pro is about €70 delivered, 6 concurrent handsets, 4 simultaneous calls. Dead easy to configure, runs beautifully, can recommend.

Probably good, but so far i can see: no analog, no isdn neither. So i cant use fax or older phones on this one (Yes i know, fax in 2017. But its highly needed where i live).

My fritzbox now run fine since months behind my LEDE. Gona keep it until i find a product satisfying me.