Thanks for posting this in the forums! That's a great question! You can force TCP on the legacy boxes since you can manually port-forward, but for the Slingbox 350 and 500, this feature is not yet available.
Has the Slingbox 350 been assigned a port? If you go to the settings tab in Watch, you can view this information. Also if you go to support.slingbox.com when logged in to your Sling Account, it will show all of your Slingbox information, including the port number it has been assigned.
Have a great day!
The Sling Moderation Team
I can confirm that a port has been assigned, but only for an IPv4 address - and that seems to be the issue here. There's no indication that an IPv6 address has been assigned to the Slingbox 350 by my routers internal DHCPv6.
The router is connected via IPv6 only! In order to reach the IPv4 Internet there's an DS-Lite tunnel where no incoming IPv4 connections allowed or possible, so none of the forwarded IPv4 ports are working, even when the local LAN devices are still running on IPv4.
Dual-Stack Lite (DS-Lite)
Because of IPv4 address exhaustion, Dual-Stack Lite (RFC 6333) was designed to let an Internet service provider omit the deployment of any IPv4 address to the customer's Customer-premises equipment (CPE). Instead, only global IPv6 addresses are provided. (Regular Dual-Stack deploys global addresses for both IPv4 and IPv6.)
The CPE distributes private IPv4 addresses for the LAN clients, the same as a NAT device. The subnet information is arbitrarily chosen by the customer, identically to the NAT model. However, instead of performing the NAT itself, the CPE encapsulates the IPv4 packet inside an IPv6 packet. The CPE uses its global IPv6 connection to deliver the packet to the ISP's Carrier-grade NAT (CGN), which has a global IPv4 address. The IPv6 packet is decapsulated, restoring the original IPv4 packet. NAT is performed upon the IPv4 packet and is routed to the public IPv4 Internet. The CGN uniquely identifies traffic flows by recording the CPE public IPv6 address, the private IPv4 address, and TCP or UDP port number as a session.