NAT6 - Unable to ping IPv6 from LAN

My ISP said they won't assign larger than a /64 so I setup NAT6.
But when I ping from a machine inside the LAN, I get:
From apu.lan (dd65:f508:508e::1) icmp_seq=1 Destination unreachable: Unknown code 5.

logread -e nat6 after completing the steps in showed no errors. IPv6 works fine on the router, named apu.

I am using mwan3-2.8.15-1 and the IPv6 connectivity is via USB tethering to a phone.
What am I missing here?

IPv6 relaying can work without NAT6:

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I had tried that, but I assume since it's giving the router an IP via DHCPv6 via android USB tethering, that does not work.