I haven't worked with IPv6 but doubt the principles are that different from v4. I don't doubt the people at Comcast are idiots. Most first-level people are not well trained and it takes forever to get to someone who knows what they're talking about. If you're on a business service though they really should know v6 exists.
Do you have a link to your router's manual on the vendor site that I could read? I would assume you'd be able to assign multiple addresses to the external (WAN) interface of your router so you can get a v6 and v4 address working simultaneously.
If the 6to4 tunneling on the router worked that would make me think you have the proper v4 and v6 addresses with Comcast and their modem is passing the traffic through to your router. Then I'd start looking at your router and server interfaces with respect to their configuration and the router IP configuration on the LAN side. And don't forget the router firewall settings. You need to make sure the firewall in the router is not blocking v6 traffic. You may have to enable that or add some rules or both to get that configured.
If the modem and router are passing the traffic you still need your server to be able to accept that. Tunneling would translate the v6 to a v4 address to get it to your server. If you're passing the v6 straight through (subject to firewall rules) then your server has to have an interface configured for v6 too.
Why is 6to4 tunneling hit or miss? Was this from your router manufacturer? Companies have been done address translation decades now. I can't immediately understand why it should be unreliable.
I hope I'm being helpful and not insulting. I don't know your level of network config experience and I'm making some assumptions based on my v4 experience. I'll do some googling. I need to know this stuff anyway as v6 becomes more and more used. I haven't really heard it's taking over yet.
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