1. In Winbox, Go to IP > Firewall > NAT

2. Click the "+" to add a new NAT rule.
3. Modify the "Chain" to "dstnat", "Protocol" to "tcp", and "Dst. Port" to "3999".
4. Set the “In. Interface” to your WAN port.

5. Click the "Action" tab, change the "Action" value to "dst-nat", the "To Addresses" to "192.168.1.200" and "To Ports" to "3999".

