Hi and welcome to ARC troubles! 
The problem is MultipleNatFound. You can’t simply turn off NAT on the router, as you found this kills everything. You need one layer of NAT so that your (one) external internet address is translated to your multiple local IP addresses that you have for your devices. However, while one NAT layer is necessary, the problem is with having multiple of them.
I’m copying Roon support’s standard response for the MultipleNatFound issue, with some of my comments in brackets:
MultipleNatFound can occur either at the local-network level (commonly as a result of two routers [which you seem to have the Synology router and the the Nokia modem in front of it]), or at the level of your service provider (in the form of carrier-grade NAT). [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. [I.e., on the ISP modem/router, create a rule to forward the port from the external IP address to the second router’s local IP address. On the second router, create a rule to forward the port onward to the Roon Core’s IP address]
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 #support: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?