You made need to do a test to see if you have a double-NAT.
This isn’t an easy thing to support as there are just too many possibilities. I will say though that Roon is using an industry-standard approach that is followed by many products. It should be easy to find solutions on line as this isn’t a Roon issue. It’s a networking issue. If your network is configured correctly, it should just work.
Best way to get assistance would be to post a diagram of your network showing the path as the network comes into your home and then leading up to your server. It is nearly impossible to provide helpful troubleshooting tips beyond the basics without understanding this. A picture would allow anyone with knowledge to immediately rule out double-NAT, for example.