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: 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.
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.
Answer: There's no indication that the ending is anything but real; the other twist in the film is heavily foreshadowed through dialogue, camera framing, and so on, none of which applies to the ending. A helicopter could have hovered above the tower and lowered a rescue basket/rope to Becky, as she's less than 100 feet from the very top. Given that the tower was probably constructed using a helicopter to position the final pieces of the structure (a common practice for communications towers of this height), there's no reason why it couldn't have held up. The tower itself is never shown to be weakened or compromised, only the ladder affixed to the outside.