... I'm not able to see anything on the GUI.
Use correct login info. Not SSH login, but Web login that you use for accessing your Antminer's interface.
Default is
root root (if you didn't change it).
Z11 frequency cannot be controlled in factory firmware.
With minerstat you can control the frequency on official firmware.
No, you cant.
Frequency settings in config are ignored and it autotunes. Please do not spread misinformation.