Serial / ASY ports are not applicable to the firewall. Firewall is only applicable to IP routed interfaces like Eth, PPP, GRE, etc.
Transport serial ports are configured to "listen" on TCP or UDP ports, for example 4000 for ASY 0. The listening port(s) can be changed via the Transport configuration.
For example, to access ASY 0 your application would need to connect to the mobile IP (or Eth IP) on port 4000. So, no forwarding is required. You will need to allow the applicable port in via the firewall.