As long as you haven’t disabled the router’s firewall/network security software or placed an active router in your network in front of a DMZ or the router’s firewall, then port forwarding with Roon is safe. Please see this thread if you’re curious to learn more: Security with Roon ARC / Roon 2.0 - #28 by Zorik_Shirman
Happy to hear ARC has been configured with your ISP’s help. Let us know if we can help further.