May I know if the ipv6 mode is NAT6 in the router/network, and the clients obtained the ipv6 global address (Beginning with 2)?

Allow outside access to your client's IPv6 address:

ip6tables -A INPUT -s <client's IPv6 address> -j ACCEPT

Allow outside access to your client's IPv6 address with TCP port:

ip6tables -A INPUT -p tcp -s <client's IPv6 address> -m tcp --dport 1234 -j ACCEPT

After rule configured:

/etc/init.d/firewall restart