Yellowstone in September

We were there in May. There is cell service near Old Faithful and up near Mammoth Hot Springs. I have Sprint and my wife has Verizon. We both had cell service in those areas but she had data more often than I did. Mammoth Campground is open year round and is very close to Gardiner, Wyoming.There is a small but nice grocery store in Gardiner that sells beer,wine,firewood and other grocery items. And the Cowboy Lodge and cafe is a good place to go for lunch. We stayed in Madison, because it was more centrally located. But the sites at Mammoth appeared to be more secluded.I would suggest staying in the park at least one night just for the experience. yes, things will be closing, but there is still enough open to make it worth your while. Have dinner at the Old Faithful Inn. Food is good and not too expensive. It is supposed to be the largest log structure in the world and the lobby is very impressive.We saw a mother grizzly and 3 cubs on the road from Norris to Canyon.