The details of exactly how DNR works will depend on the State. In Alaska it is called "Comfort One" and you enrole through your physician. Here is how it works in my state:

The Alaska Comfort One Program: Information for Health Care Providers

Note that it states explicitly:

"The protocol (contained in state regulations)
requires that the physician or other health care
provider (EMT, Paramedic, etc.) immediately
proceed with patient assessment and care,
including administration of CPR, until the
patient’s identity is confirmed and the patient is
determined to have a valid DNR order."