IPv6 packets on WAN are not forwarded/routed to the client

I see the ICMPv6 echo request and reply are forwarded properly.
On the other hand, ICMPv6 neighbor solicitation seems to fail.
Check the IPv6-related firewall settings on the source host.

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I am not sure what to check, it is a fresh installation of Ubuntu 20.04 and there aren't any specific firewall rules:

ip6tables -L -n -v
Chain INPUT (policy ACCEPT 71 packets, 9488 bytes)
 pkts bytes target     prot opt in     out     source               destination

Chain FORWARD (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination

Chain OUTPUT (policy ACCEPT 139 packets, 13408 bytes)
 pkts bytes target     prot opt in     out     source               destination
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Can this ubuntu ping the router?
If you move the ubuntu to the lan interface, does ping work?
If you move a working host from the lan to the DMZ interface, does it still work?

I think the issue is solved and it was not OpenWrt related!

Sorry for that and many thanks to all of you, who helped me tracking it down.

The reason was my poor KVM/libvirt setup. :frowning_face:
All clients connected to the OpenWrt LAN interface was either real hardware or kvm guests with bridged interface and worked well. But all clients (virtual guests) who had IPv6 connectivity issues were connected to the DMZ interface of OpenWrt via mactap in KVM/libvirt. If you want to use this configuration you had to edit the interface definition of the guest and add trustGuestRxFilters="yes"

I found the solution here: IPv6 with macvtap and libvirt

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