Was told I can have static v4, or v6, not both (moved to this forum)

This was posted on the 'static route' sub-forum, and I am thinking that perhaps it is better for the v6 forum. So here goes...

Right now I have a small range of static IPv4 addresses from Comcast (Netgear CG3000DCR). I've been using these for years. However, I want to enable IPv6 on a single server, to start migrating it to full dual stack support (both v4 and v6), anticipating the day when it will have only v6.

The person on the phone indicated I can have either static v4 address block, or a static v6 prefix, but not both at the same time - that Comcast would not provision both to be enabled simultaneously.

Is that truly the case?! I just don't feel comfortable using a dynamic address for v6, because pointing a DNS record to a dynamic address, trying to get a reverse entry set up for my mail server, etc. - that just is crazy to do with a dynamic address.

As a result, I feel I am stuck in IPv4-only land for my server - which is really, really odd. I would think I should be able to set up dual-stack on my server, if I choose to.

In particular, here is what DOES work:

a) I use the EUI-64 address on my server, using the dynamic /64 IPv6 prefix I learn from the Comcast gateway. For example, my /64 prefix from the gateway is: [2601:647:wxyz:a400] (I am hiding a portion of the prefix as 'wxyz' in this email). My EUI-64 address is: [2601:647:wxyz:a400:201:c0ff:fe0d:753e]. This works fine. I can ping6 from this, ping6 to this, and I can even serve up web pages from it via Apache HTTP. 

b) I pick an address of my own from the /64 prefix from the gateway such as: [2601:647:wxyz:a400:10::1] and use that. I can ping6 from this, ping6 to this, and I can even serve up web pages from it via Apache HTTP.

What does NOT work:

c) I pick an address of my own within the static prefix range listed on my account page, which is [2603:3024:WXYZ:4600::/56] (again, I am hiding 16 bits of the address as 'WXYZ'). Traceroute6's to it show that packets reach a Comcast router within 30 miles of my home, but don't get any closer. (The traceroute6's for cases (a) and (b) go through the same 30-mile-away router, but stop there).

Is there no way to get a static IPv6 address from Comcast, at the same time retaining a static v4 address? Or, should I *ignore* the /56 indicated on my account's info page (the 2603... one), and just use the range my Netgear CG3000DCR is showing (the 2601... one), and treat that as my 'official static address prefix'?

Thanks!