Hello,
Is the IP address registered in the DDNS a public one and is it the same as the one you can see from the output of ifconfig wwan0 ? If it is in 10.0.0.0/8, 100.64.0.0/10, 172.16.0.0/12 or 192.168.0.0/16 then it is a private one and cannot be reached from the outside world.
If the IP address of the router is public the first point to check is if the port (51820 ?) is open in the firewall. If it isn't add a rule to allow it.
If the handshake still fails after that then the most probable cause is a key mismatch between the two routers.
Regards,