Interestingly the information wireshark gives is:
7540 79.991748 2a02:348:82:cb69::6 2406:e001:1:2301:250:56ff:fe92:78 ICMPv6 106 Echo (ping) request id=0xce39, seq=1, hop limit=242 (no response found!)
so it wasn't able to find a response, even though the IP is available behind the router.
I've just checked the routing packet and I've got the reply from the router stating that it was administratively blocked... I do think the 7390 is not allowing IPv6 forwarding!