Sorry this is happening to you
We had exactly the same with my dad.
I previously thought that a dnr was giving up on someone, until it was properly explained to me. Basically resuscitation is applying extreme force to someone’s chest, which is likely to break ribs and possibly puncture a lung. So it is likely to put the person in a lot of pain. Also, if their heart is stopped for a period of time it is likely to give them brain damage. Therefore, if they survive they are likely to be in a very bad state, with who knows what sort of death awaiting them, and if they die during the procedure they are likely to die in pain.
If there is a dnr in place and their heart just stops, then they are much more likely to experience a pain-free and peaceful death.
Your dad may have said he wanted to live as long as possible, but what sort of life would that mean for him? Also, if his heart stops, he would be effectively dead then, and you would be bringing him back from that, but for what?
I used to think that a dnr was playing God, but now I think that it is playing God to keep someone alive who should really be dead.
My dad passed away peacefully by the way.
I am sorry you have to make this decision.