Introduction
My front deraileur was stuck in the highest front gear for half a year, I hand no idea how to fix it but just found out it was extremely easy.
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            On the lefthand side of your Stromer, bend over to have a look below the gear shifter. Notice it has a switch. First take note of the position of the switch and flick it to the other side. Try again and see if your problem has been fixed. 
 
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                            Conclusion
                          
                          If it didn't help, put the switch back in its original position