I strongly suggest that, if you go digital, you steer away from mobile/pc/table. Having trouble retaining information may be directly linked to social media use, multitasking, mental fatigue, poor diet, poor sleep. Physical books and e-readers help you because you don't have the chance to switch to another tab.

Before you try any complicated method, try to set up the room where you read to be quiet, turn off notification or leave your phone in another room, have a soft light, no background noise. You basically need to be quiet and concentrated. It's normal to forget some stuff you read, I read a lot and some books I completely forgot after a month. But at least you'll give your brain quiet time and you get used to focus.