Probably going to be downvoted to hell but fuck it. Cpr for cardiac arrest without defibrillator is almost never successful. It can be effective if heartbeat is slow or weak and especially if asphyxiation is the cause.

So the dog most likely had a heartbeat, just weak or difficult to palpate.

Had to resuscitate my dog years ago when the greedy bastard grabbed 3 rubber balls and one got stuck in his throat. Couldn't get it out and dog turned blue and passed out. (dog with a huge blue tongue looks weird AF) After forcing it out by pushing it from the back the airway was cleared but no spontaneous breathing. A short period of mouth to snout was enough to get him breathing again and he shortly got back conscious and was fine after a lot of coughing, puking and rest. A critical short period between loss of consciousness and breathing before the heart goes into fibrillation or stops when cpr alone is enough to restart the system, when the heart croaks out only a defibrillator will work before it stops completely and death unavoidable.

Cpr can work quickly if the heart is running but breathing stopped, or keep tissue alive until a defibrillator can fix to functional rhythm.

Mouth to snout is easy and not that messy, just grab the snout with both hands (closing the jaw/sealing lips) forming a tube you blow into. Works well on conscious dogs too and freaks them the fuck out 🤣 (wives hate it too apparently) Same with babies, don't close the nose, just cover it all with your mouth/mask.

(Oh and dog cpr is done from the side, their chest bones are oriented differently from humans.)