I'm not sure why port-forwarding would help here. Port forwarding is mostly useful for incoming traffic, unless you're on an enterprise network with much more complex network configurations.
Tell them the service is not up to par with your expectations. You're paying for 20 Mbps upload, and you're not getting that. Tell them exactly what you're trying to do, and that that requires a stable upstream connection, and you're not seeing a stable upstream connection.
Also, there are rumors that some ISPs try to cheat speedtests, especially Speedtest.net. You might want to check those results against other speed testing services. Here's a list of options: