I have used the didi app before in Oaxaca and I had trouble using my CC. I wasn't sure if it was the poor cell service or something else but there is also an option to pay cash to the driver. It works exactly like uber except if you pick the cash option you will pay the amount at the end of the ride. The amount is shown on your phone as well as the driver.

You will have such a wonderful time in Oaxaca - the food and culture there is very special. I was there in May and am going back in Sept.......