I can get RoonArc port forwarding to work through my Spectrum SAX2VIS router.
My problem is that despite reserving the IP address and ports that the Roon server is using via the Spectrum app and router resulting in successful and awesome use of Roon Arc, every 2 hours Spectrum changes the IP address of my Roon server.
So, essentially, every time I end up using RoonArc, I first have to figure out what the new IP address Spectrum gave it and then configure the router.
Spectrum support told me the only way I can reserve a static IP address and port, despite the option existing in their app, is to incorporate an at home business and buy one of their business plans.
I am guessing someone out there has figured out how to work around Spectrum and their consumer grade routers.
As you seem to have a separate modem anyway, it should be no problem to ditch the router and get your own good one.
I am personally very happy with my private Fritzbox (get the most modern one with the latest wifi v7) but I’m sure others will have their own pet recommendations.
I would second the suggestion of getting another router and just keeping the modem. While we (support) can’t suggest any specific router, most consumer-grade (non-ISP) routers should be adequate. Our community has recommended routers in the past here: