It appears that they have the documentation for several ZTE ZxDSL series modems. I think you should check that site for the instructions on how to set up port forwarding. Pictures included.
To give you some background on port forwarding....
The reason why you are investigating port forwarding is because you want a public Internet host to access a computer with a private IP within your local area network. Since that computer has a private IP, its behind your router and inaccessible by Internet hosts.
You can use port forwarding to get those packets from the public Internet into your network. The way it works is that you provide your public IP address to the source Internet system. The connection is made to your public IP on a certain port. Your router will use the port forwarding configuration to take that packet and "forward" it directly to your internet host. The public Host does not know that it is communicating with a host on a private network. The router takes care of this mapping between the two systems.
So what you need to do is map a public IP/port --> to a private IP/port.
The trick here is to make sure that your public IP does not change. If it does, then you will need to use a service like noip.com which provides you with a DNS hostname. Then, you have to run an application on your network which detects the public IP address and updates this host name. You then provide the host name to the internet hosts that is trying to access your system. every time your public IP changes, the DNS host record gets updated.
If you want to check your public IP address from a web browser, just visit this site, mySourceIP.com.
If you can get the exact model number that you have, I can take a look at the reference documentation and provide some additional feedback in case you have another question.