The two codes may not be related since one is for the transmission and the other one for the engine. The P0300 code is triggered when the engine is misfiring. Misfiring problem can be caused by a range of components, spark plugs, injectors or coils. You should start by replacing the spark plugs.
P0705 HYUNDAI - Transaxle Range Signal Malfunction
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P0300 HYUNDAI - Random/Multiple Cylinder Misfire Detected
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