Ipv6 handoff question - can't route from internal subnet

I am being assigned a /48 from my ISP, but we are having some trouble with it. I've been assigned a /48, with an address block similar to this (not my actual address) - 2001:abcd:aaaa::/48 . They've assigned their network interface (their handoff on their router) as something like 2001:abcd:aaaa::1/48.

Now, I need to break up this /48, so I made the external interface a /64 and assigned my router's wan port to be 2001:abcd:aaaa::2/64 . I can ping the ipv6 internet through this address.

For my internal interface I used 2001:abcd:aaaa:1111::1/64 . However, this internal subnet has no internet connectivity. I believe my cisco config has been set up right (ipv6 unicast-routing, ::/0 default route set to 2001:abcd:aaaa::1 , etc).

Now, I'm thinking the problem is that their handoff interface is configured as a /48. Their router thinks the entire /48 subnet is directly connected, and will never try to route any packets through my router. Instead, packet captures seem to indicate it is trying to use neighbor discovery to find addresses like 2001:abcd:aaaa:1111:1 (which is being used internally).

The ISP is telling me it should work fine. This is how they should be doing the handoff, and their interface has to be a /48. I'm telling them it should really be a /64.

Then again, I'm not ipv6 expert. A sanity check here is appreciated.