You can't directly control the two dials on the left, they are a display that represents the current firing solution.
1) Make sure you have an aft tube selected. You will notice that the display changes if you have forward versus aft tube selected, as the firing solution is different.
2) Make sure the target is roughly behind you. The bearing as you look at it through the periscope should be between 135 - 225 or so. You can't fire aft tubes at a target in front of you and vice versa. The torpedos can only turn so much after leaving the tubes. If it's not behind you turn your sub until it is.
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