Question: When the girls realize that cellphone signals are available at the base of the tower, why can't they fly the drone down with the cellphone as a load? Is the payload too heavy?
Question: There is a running theory that Becky actually did not survive, and that her father was imagining reuniting with her was actually his brain reacting to him seeing her body, whether or not first responders were on scene. So, if Becky actually was rescued, how could the helicopter have gotten close enough to reach her without the satellite collapsing due to the helicopter's downwash or the propellers hitting the tower itself?
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.
Question: When the girls got stuck at the top of the tower on the first night, why didn't they think to climb to the very top to screw and unscrew the light bulb back and forth in an SOS pattern at alternate speeds? That surely would have been seen as out of place, and the SOS is universally known as a distress signal that would have alerted motorists and people on the ground. No doubt that would have been strenuous to perform, but at least they would have been trying everything to get help.
Answer: To add to the other answer, while "SOS" as a distress signal is widely known, the Morse code equivalent (... - - -...) is not. So, unless they were educated in such signals, anyone seeing it probably wouldn't interpret it as a deliberate call for help...even law enforcement in this day and age couldn't be guaranteed to understand it correctly.
Answer: They don't have the backpack at that time?