Factual error: The events of the movie occur over 3 days, during which time a cellphone is used repeatedly, including for piloting a drone. There is no currently available high-end cellphone which would continue to operate this long without charging.
Factual error: While the "life hack" of charging using a North American plug into a light socket might work, the use of a wedding ring to shorten the distance between the charger prongs and the fixtures, as depicted in the movie, would definitely not work, as there are two prongs that both need connection. All the wedding ring would do is to short out the connection.
Factual error: The drone used in the film would not be capable of the range depicted. For this small drone, using a usual 2.4GHz WiFi signal, a range of less than 1km would be expected. In addition, it is impossible to pilot a drone over 1km away without using hi-end optics to see it, as it is too small to see, let alone pilot. While there were binoculars on the platform, due to the plot twist, these were not able to be used.
Answer: They don't have the backpack at that time?