Create a variable in your game instance BP(it is a string in my case. You can make it a float if you like) and then call and set it in your widget BP like so:
Then in the widget BP again, call and get the variable to set the slider’s value:
Game instance will pass the variable between levels as long as the game is running. Of course, if you want to save the slider position and the value after you restart the game you need to create your own save system.