I need help configuring IPv6

[Nathan Van Ymeren <nathan.vany@gmail.com>]

I have scoured the internet for days, but I cannot find a coherent
guide that is understandable to me for configuring a static ipv6
address on debian 11/bullseye.

I have been given the following from my datacenter:

Assigned IP range: 2604:xxxx:xxxx:xxxx::2/64 -
2604:xxxx:xxxx:xxxx:ffff:ffff:ffff:ffff
Subnet Mask: /64
Gateway Address: 2604:xxxx:xxxx:xxxx::1/64

I have read countless guides but it is still not clear to me how to
get ipv6 working, because no matter what I try I get "network
unreachable" when I try to ping6.

IPv4 works on eno1.  All I want is to just assign 2604:blahblah:2 to
eno1, but so far have been unable.

The relevant portion of /etc/network/interfaces is:

iface eno1 inet6 static
        address 2604:xxxx:xxxx:xxxx::2/64
        gateway 2604:xxxx:xxxx:xxxx::1

As far as I can tell from reading the debian wiki and whatever else,
it should work like this, but even ping6'ing the gateway gives "ping6:
connect: Network is unreachable".  I can ping the gateway on its
link-local (fe80) address, but that's all I can do.

Please help!