Hi @Richard_Rodell,
Depending on your geographic location, you may have to pursue workarounds to Vodafone’s carrier-grade NAT (as @Rugby mentioned) until Roon’s internal efforts to implement IPv6 compatibility for ARC bear fruit.
Many Vodafone user’s have successfully configured ARC, either through contacting Vodafone support, tweaking/replacing their Fritz!Box and settings, or setting up a VPN. I recommend you check this thread out: Vodafone Germany w/ ISP-Supplied Modem/Router (Fritz!Box or Touchstone): Ongoing Discussion
Thank you for your patience.