I just tried using Sob’s solution where he used VRRP to put multiple DHCP clients on one interface. Unfortunately it doesn’t work because the DUID of the DHCPv6 client is identical to for every DHCPv6 Client. Again, not blaming Mikrotik here. That is how the DUID is supposed to work, but it’s still disappointing we can’t edit the DUID (it’s even more disappointing that a behemoth like AT&T doesn’t understand how to implement IPv6 though). Even if I used multiple physical interfaces to connect to the modem, the DUID would be the same across all of them, meaning the additional DHCPv6 Clients would not work.
I guess I will have to wait until either Mikrotik allows modifying the DUID on the DHCPv6 Client, or Arris updates their modem to respect prefix hints.