If you raise the minimum wage high enough so that it provides for a liveable wage in a major city and are able to enforce it, then the amount of people willing to pay migrants for work will collapse and the migrants will stop coming to the city.
So you either accept the conditions and wages the migrants are being paid, or you eliminate the very reason migrants travel to cities in the first place.
I want everyone to live in comfort, but it's much more complicated than "just open all the borders and raise minimum wage".