Mikrotik RouterOS DHCPv6 stuck "searching" for prefix or address

I am setting up IPv6 on my Mikrotik hEX (RB750Gr3) with RouterOS 7.15.3, but I am not able to get an IPv6 prefix or address. It is stuck in "searching" state forever.

I understand that Xfinity broadband customers are supposed to be assigned a /64 IPv6 prefix, so this must be a configuration problem.

Here is what I did:

/ipv6 route
add disabled=no dst-address=::/0 gateway=ether1 routing-table=main

/ipv6 settings
set accept-redirects=yes-if-forwarding-disabled accept-router-advertisements=yes disable-ipv6=no forward=yes max-neighbor-entries=4096/ipv6 dhcp-client
add add-default-route=no dhcp-options="" dhcp-options="" disabled=no interface=ether1 pool-name=comcast-ipv6 pool-prefix-length=64 prefix-hint=::/64 request=prefix use-peer-dns=no

/ipv6 firewall filter
disable 4
add action=accept chain=input comment="Replaces defconf rule for DHCPv6 client prefix delegation. https://forum.mikrotik.com/viewtopic.php\?p=1019432#p914681" dst-address=fe80::/10 dst-port=546 log=yes log-prefix="UDP 546 (DHCPv6) - " protocol=udp

I based this setup on advice from the Mikrotik forums:

• https://tangentsoft.com/mikrotik/wiki?name=IPv6+with+Xfinity
• https://forum.mikrotik.com/viewtopic.php?p=914681
• https://forum.mikrotik.com/viewtopic.php?t=201029
• https://forum.mikrotik.com/viewtopic.php?p=1012740
• https://forum.mikrotik.com/viewtopic.php?t=183759

This is all very standard except the ::/0 route and the firewall rule, which apparently are necessary due to how Comcast set up their DHCPv6. You will see that I did enable logging for the custom firewall rule to check if it was working, but I have not seen any log entries for it.

So, what's the problem? Is my configuration still incorrect?

Note that I am not a networking expert and my understanding of how DCHPv6 works is rudimentary.