Just roasted a chicken, olive oiled the skin, applied chicken rub and placed the chicken in a preheated 350 degree grill. It came to temperature in about two hours. Skin was very tough! What does it take to get a tender tasty chicken?
If you want crispy chicken skin, you need to begin with dry chicken and a hot oven. When I roast a chicken, I spatchcock first. Then salt liberally, and place in the refrigerator, uncovered, for at least 24 hours, and up to 48 hours. This removes moisture from the skin. I roast at a high temperature: 450F (232C) for about an hour and 10 minutes, but check the temperature in the thigh for best results.
I suspect that the low heat, wet and oiled skin, and likely overcooking caused your skin and chicken to be tough.