DNAR forms are a tricky area in medicine with a lot of misconceptions around them.
For your immediate situation, as bails says above, I think your best course of action would be to contact the PALS service at the hospital that treated your grandfather. IME they are very good at resolving situations like this. It’s worth contacting your grandfather’s GP as well to make his wishes clear, in case the GP has amended his records to note the DNAR.
As an aside, and contrary to popular belief, nobody has an automatic right to have CPR performed. It is ultimately the doctor’s decision whether that treatment is appropriate, not the patient’s. That being said, if a person is of sound mind then that decision should always be discussed, even if it is to say that CPR would not be offered. It sounds like that didn’t happen with your grandfather, which is disappointing.
All that being said, the default course of action where there is any doubt whatsoever is to attempt CPR.