Sadly, due to my past experiences with different ISPs here in Germany regarding other issues, that’s hopeless. Maybe I will give it a try.
The cause why it doesn’t work is addressed and explained in the topic below. This is exactly the common configuration I and many others have and not only specific to my ISP and router. Look here:
What is CGNAT and why it doesn’t work with ARC
Looks like there’s some homework for Roon waiting to be done to get ARC working for all users 
Thank you very much for the suggestion, Axel. I have this Vodafone tariff and router for private customers (without static IP). Your suggestion is for business users. Let’s see if Vodafone allows the switch …
No static IPs for residential users from Vodafone 
The IPV6 DS Lite Tunnel Vodafone is using prevents ARC functionality over IPV4:
Vodafone Germany and Fritz Box 6690 Cable: 502 Error in Port Forwarding Diag
@Axel_Lesch
I will try to change. It’s not possible to do it online on the Vodafone site. The costs are the same as my residential account.
No need to get a business account… try this:
SOLVED: Do I really need a static IP to make Roon Arc work? - Support / Port Forwarding Help - Roon Labs Community
@ JST1963
Thanks for the hint. Unfortunately I cannot put my router in bridged mode because it also handles telephony. The Problem is my ISP (Vodafone) which handles IPV4 internet connections via a IPV6 DS Lite tunnel. These are derived internally (NAT) from the external IPV6 address. Therefore I have 2 NATs for internet connections. The only solution is a public external IPV4 address which my ISP grants only to business accounts and not to residentials. Otherwise I will have to wait until Roon offers a solution for such DS Lite Tunnel configurations over IPV6.
Antonio, my solution is not putting everything in Bridge mode. I solved it by using port forwarding in the two modems. Please read until the end of hat posting…
Does Vodafone support PCP (port control) if so you can create the port forward locally and this is then actioned within Vodafone for you, getting around the ipv6 issues.
The problem is Dual-Stack Lite / DS-Lite. You get a “Fake” IPv4 and a real IPv6. With fake I mean the IPv4 is not accessible from the internet. Only the real IPv6 is.
But if the application is not made with this in mind, it will use the IPv4 and be happy with it. It doesn’t know that only the IPv6 is working.
In situations where a IPv4 as well as a IPv6 is available, applications should prefer the IPv6 and ignore the IPv4.
Where do you see a joke?
developers writing apps to prefer ipv6 over ipv4 (or even support ipv6)
I know. But dreaming isn’t forbidden yet, is it? 
Oh no of course not, I keep dreaming that Roon will start supporting their customers and not leave it to a forum
@ Ben_Humpert
That’s exactly the situation and a viable solution in ARC/Roon. But as long as ARC awaits a real public IPV4 address (not NATted as here) for access from the internet, it won’t work in environments that only can provide public IPV6 addresses as with DS Lite.
I do have Vodafone Cable and a FRITZ!Box … but because of DS-Lite and Plex being “too stupid to figure that out” too I bought my own FRITZ!Box with which I’m able to get Dual Stack, not Dual Stack Lite. That’s why my Plex as well as my Roon ARC works “as intended”.
Hi Chris. Welcome to the forum. Might be a good idea to get the lay of the land before you take a bulldozer to it. 