There are two height limits: out of the box, Tello is limited to 10m. This limit can be increased up to 30m by using TALS (available for Android and iOS). The 30m limit seems to be fix in Tello firmware and I have not yet seen a way to increase it. IMHO, it would not make too much sense anyway because Tello's vision positioning system does not work well anymore at these heights. Additionally, winds are almost always considerably stronger up there, making it hard to keep control.