How do I cut an unwanted frequency from recording?

You would need a DeNoiser to do that. Wavelab Elements (which features Cubase integration) has a plugin called RestoreRig, which includes a DeNoiser module:

This video explains how the Cubase integration works, and gives a few examples of how it can be used:

Alternatively, you could just upgrade to Cubase Elements ($49), which includes high pass and low pass filters in every channel. It wont give you results as good as a DeNoiser, but they’re better than a basic EQ for removing unwanted noise.

I explained the main differences between Cubase AI and Cubase Elements here: