I read How does IPv6 subnetting work and how does it differ from IPv4 subnetting? but my question was not answered there.
I'm upgrading our IPv4 network to IPv6. Currently, our NAT gateway splits our one IPv4 address into 2 private subnets, our main subnet and an isolated guest subnet. I want to continue this practice of having 2 separated subnets under IPv6. I read that I should not use a prefix larger than /64 because it will break things, but the delegated prefix my router is picking up (I believe via DHCPv6) is a /64 prefix. So somehow I want to get a /56 prefix automatically assigned (suggestions welcome), but even after I get that, it is still going to be dynamic, so my question is how do I set up 2 subnets based on this dynamic allocation?
I am used to using NAT and statically configured IPv4 private subnets. Now I am going to have to manage 2 public subnets with a firewall between them, but I don't see how I'm supposed to configure a RouterOS router to say "combine the /56 dynamic prefix with this 8-bit static subnet identifier to create /64 subnet". How do I do that (or what should I do instead)?