Custom ports are generally not available on the Jetpack devices by default. The main reason is that all Jetpacks tend to sit behind the NAT firewall where NAT security policies block custom traffic. I do not have access to a mhs291L to confirm but its safe to assume that this device is treated like the other Jetpacks.
Here are the work arounds:
1. Get a static IP address for around $500 from VZW.
2. Get HomeFusion. HF comes with a free static IP.
3. Look into VPN tunnels. A VPN will route your traffic through the VZW NAT to a server where the ports are open.
4. Terminate your service and look for another provider.