In this instruction we will  forward  3389 TCP port to local Server address 192.168.1.200

1) Go to IP -> Firewall -> NAT

2) Click the  RED “+” to add a new NAT rule. Set the “Chain” to “dstnat”, “Protocol” to “tcp”, and “Dst. Port” to “3999”. Set the “In. Interface” to your WAN port. ( In Interface  is IMPORTANT).

3) Click the “Action” tab, change the “Action” value to “dst-nat”, the “To Addresses” to “192.168.1.200” and “To Ports” to “3999”.

4) Click APPLY

Done  all  traffic comes from internet on port  3389 will forward to local server  192.168.1.200 on same port

P.S.

For Asterisk  you should forward:

5060 UDP — as  SIP (signaling )

10000-20000 UDP  -as RTP (voice traffic)

If you need  extra services:

22 TCP – as  SSH

80 TCP -as  HTTP

 

 

-----------------------------------------------------------------------------