Setting Airplane mode requires a careful procedure, to be sure that it doesn’t happen by accident. Here’s what works for me.
Find a quiet place (so probably not just before the plane takes off!).
Take the band off and hold it close to your ear, upside down (ie with the orange section upwards).
Press and hold one button until you hear a very short, quiet buzz (the “blip”). If you are counting seconds, expect to get closer to 8 than 7.
Immediately release, press, release. You should hear 3 more blips, but the first may be masked by the click of the button so if you hear 2 but not 3, don’t worry.
If you are doing this within sight of the base, don’t worry that the “Band connect” light doesn’t go out straight away: that won’t happen until the next routine transmission is due.