Getting 6in4 working via VPN

[wrtpoona]

My OpenWRT router sends all IPv4 traffic to a Wireguard VPN hosted in GCP.
Sadly, the GCP instance does not have global IPv6

Since the 6in4 traffic is also sent via the VPN, my OpenWRT router cannot setup the 6in4 connection once the VPN is setup.
I assumed this was because the VPN server was not forwarding Protocol 41 traffic back to the router. I see traffic sent on OpenWRT's 6in4 interface but 0 received.

On the VPN server I tried (to no effect):

Code Select Expand

iptables -A INPUT -p 41 -j ACCEPT
iptables -A PREROUTING -t nat -p 41 -d $VPN_PUBLIC_IP -j DNAT --to-destination 192.168.1.10 # OpenWRT router internal IP

How can I get this working via the VPN?