When I am putting ore in or out of the smelters at times, the on off button pops up. I cannot get rid of it besides backing up quite annoying. Also at times I cannot put anything into the furnaces
To take ore out of the furnace, you need to aim at the ore and press the interaction key. Make sure your crosshair turns into a small hand. If you see a circle with three dots, you're aiming at the furnace itself (pressing the interaction key brings up the small context menu in this case).
To put ore into the furnace, make sure you see a small "preview" when aiming at the individual slots (having the ore equipped)