Lead-free solder requires an extra hot iron to melt. It will melt much more easily if you add a little leaded solder. This will not only increase the thermal contact between the iron and the lead-free solder, but will also reduce its melting point as the solders mix. Use a solder sucker or solder wick to remove excess solder before reconnecting.

On reassembly, double check that this connector is fully engaged and pressed home (force is not required). If not fully engaged, you may find the drone appears completely dead. At the end of a couple of hours of intensive work on your precious toy this can cause an unpleasant sinking feeling in the pit of the stomach.