I will try to help you. I assume that your Roon Core 2.0 is working well on your Synology and that you can play music with Roon Remote 2.0.
Can you give information about your network gear. What do you have? Modem, modem/router, router, etc.
Furthermore let us work from outside (internet) to inside (the NAS in your LAN). You wrote that you have setup portforwarding for Roon ARC. Are you sure that the (right) port is indeed open to your LAN? Please check with GRC | ShieldsUP! — Internet Vulnerability Profiling