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My MacBook Pro's trackpad stops working when I connect an external keyboard, which prevents me from properly configuring the external keyboard

I'm trying to set up a new Ergodox EZ keyboard. Whenever I plug in the keyboard's USB cord, the MacBook's trackpad ceases to function. It responds to absolutely no input at all. This makes it impossible to configure the keyboard properly (I have to go through a wizard so the computer can figure out the keyboard uses an ANSI layout). If I disconnect the keyboard temporarily, I can press next, plug the keyboard back in, and then continue with the wizard, but at the final screen I need my mouse again, so I have to disconnect -> click -> reconnect, at which point the wizard reappears from the beginning claiming that the keyboard could not be identified.

How can I fix this?