Newbie Webserver Behind the Astaro 320

Dear all,

I have an firewall Astaro 320, after this firewall I have a Webserver (IP 192.168.1.78), This webserver go through port 55555.

From internet outside, I can access this webserver via url www.dht.tdns.com.vn

But from internal (after this firewall) i can not access this webserver via url www.dht.tdns.com.vn .

Please help me how to configure to allow people after the firewall can be able to go to this url www.dht.tdns.com.vn ?!

:(
Tons of thanks,
Ban