Cannot use DNS from outside

I just set up the DNS (private IP) behind the gateway router and already set
the port forward 53 to this private IP.
In gateway router, i disable "Block WAN request" in Firewall section.
However, this DNS cannot be used from outside and I can use it in internal
network. Is there any configuration I am missing? Please kindly help.
Thanks!

I solved it now. i have to add port forward 53 in UDP, but not only TCP.

"May" <M@y> ¦b¶l¥ó news:eSUx% ¤¤¼¶¼g...
>I just set up the DNS (private IP) behind the gateway router and already
>set the port forward 53 to this private IP.
> In gateway router, i disable "Block WAN request" in Firewall section.
> However, this DNS cannot be used from outside and I can use it in internal
> network. Is there any configuration I am missing? Please kindly help.
> Thanks!