There are a couple of ways to do this. You can do direct blueprint communication using casts to transfer the public variable around, or use an interface to pass the variable between blueprints. There is also some other ways, but those are only ways I am familiar with. If my information is faulty, someone please correct.
For the first method, using direct blueprint communication. You can cast from the widget to where you house the public variable you want changed( game mode would work well). So now any time you change your widget slider it directly changes the variable on the game mode. Then create listening events/functions on your level blueprint that links all your actors to update whenever the variable on the game mode changes.
For the second method, using blueprint interfaces. You can create an interface and setup a basic function that moves the data from one BP to another. Simply add the variable to input/output sections. Attach the interface blueprint to your widget, and your level blueprint. Setup at the end of your widget slider function to send the data to your other BP and setup a function that listens for a change in variable and updates.
Documentation on both methods:
Update if you are still having trouble, good luck.