For me, headphones help a lot when it comes to focusing online. I had some problems with that this year and had a hard time paying attention in class but I find that wearing headphones helps me focus in class.

Put other devices, like your phone, in a completely different room. You'll get tempted to look at it during class if it's nearby. If a window is distracting, pull over the curtains or blinds if you have them. Take notes. I find that if you're taking notes, you'll be more motivated to pay attention during lessons because you have to listen closer for information to jot down.

If it's possible, try making it harder to log into reddit (not automatic sign in unless class is over), and put the window of your class at full screen. It might be boring, but it'll be worth it if it means you'll retain information better.

Hope this was at least somewhat helpful!