Hello, I've just bought a ZTE ZMax 2 (Using It With T-Mobile) . I originally set the display sleep time in the settings to 1 minute, but then all of a sudden I notice my screen's turning off too early again so I go to increase it again. But this time the only option I could select is "15 Seconds," before I had a range from 15 seconds to 30 minutes, I've googled this and have found that if you have Power Savings mode on it'll default to 15 seconds. I didn't and don't have it on and still can't change the display sleep time. Thanks for helping.