It appears your ISP is a joke… maybe you should post their name here for everyone to avoid it.
ISP manages router, won’t do anything consumer requests, does not do IPv6, how long will that survive?
Teredo tunneling works independent of any involvement from a router - in fact, that’s the very case it was designed to deal with. It’s designed to tunnel out to a server somewhere in the world and use whatever UDP port number the session appears to come from. Unless your ISP is actively blocking teredo traffic, it should work. It’s very slow, though, so don’t expect gaming to work over it very well.
Teredo has to be supported on the end device, not on a router, and I doubt that the Xbox supports that.
You should ask your ISP to put their device in bridge mode and let your own router get the public IP address directly. It’s also possible they’re operating with CG-NAT (or poor man’s CG-NAT - a single firewall somewhere that just does nat for everyone)