There is more than one network address translation (NAT) layer active, preventing port forwarding from working although the port forwarding on the router is configured by UPnP.
I’m copying Roon support’s standard response for the MultipleNatFound issue:
MultipleNatFound can occur either at the local-network level (commonly as a result of two routers), or at the level of your service provider (in the form of carrier-grade NAT). [Comment Suedkiez: Or it could be both at the same time]
If your setup involves an ISP-provided gateway (modem/router combination) and your own third-party router:
In the web administration interface of the ISP-provided gateway (modem/router combination), enable Bridge Mode or equivalent, where the ISP-provided gateway does not have DHCP routing enabled.
Alternatively, if you have already created a manual port forwarding rule in your 3rd party router, you can add an additional rule to forward the port through the ISP/second router.
If you only have one router in your setup or your modem is already in Bridge mode, please take a look through our list of known router and internet service provider solutions, as other users may have already encountered the same situation: ISPs and Routers: List of Known Solutions and Workarounds
You can reach out directly to your service provider to ask if they support port forwarding; this question will often enough to prompt them to explain whether or not the carrier-grade NAT they’ve implemented can function with ARC.
More specifically, you can pass along the following questions:
Have you implemented carrier-grade NAT for my account level?
Have you fully implemented IPv6, or do you have IPv4 addresses available?
Can I request a static IPv4 address to support port forwarding? [If the support people are clueless: Every online gamer needs the same thing, e.g. for a PlayStation]
Are there any ports you have reserved at the ISP level I should be aware of?
If you’re unable to locate an existing solution in our ARC: Port Forwarding Resources subcategory, please reach out to the Roon support team and include the following information:
What is the make and model of your modem and router?
Do you have any additional network hardware, like additional routers or managed switches?
Who is your internet service provider and what is your geographic region?
Is your Modem configured in Bridge Mode so that it operates only as a modem or do you have the ports forwarded on both?
