I'm assuming you've contacted 911 / emergency services since you know that the ambulance is 20 minutes away. In that case, the dispatcher will step you through an emergency diagnosis and if such an extreme action is warranted either they will put you in touch with a medical professional who can instruct you on safe procedure, or they will be a qualified paramedic and instruct you themselves. However that is EXTREMELY unlikely, tracheotomy are almost never warranted (outside of television) in emergency situations, as stabbing someone in the neck is not a trivial thing to do. In my region the procedure isn't even taught to first responders (Edit: I was half wrong, paramedics still learn it but EMTs do not) as it's long been surpassed by modern intubation techniques and treatments like fast-acting anaphylaxis medications.

In short, follow the guidelines you are taught in your first aid class and contact emergency services. Don't stab someone in the neck.

So, if the person has completely stopped breathing, and ambulances are 20+ minutes away, should I limit my response to attempted CPR?

The answer is: YES