Hardware/Software:
P4P : 01.05.0600
Controller : 01.04.01.00
DJI GO 4 : 4.2.14 (iOS)
Neither in 2.4 GHz, nor in 5.8 GHz, "Custom" is selectable.
"Auto" is always enabled!
DJI Forum : Auto/Custom Channel Mode in iOS w/o Region change possible?
Besides this ... I noticed that the 2.4/5.8 GHz setting is maintained when the drone power is cycled.
If the controller is power cycled, the setting reverts to 2.4 GHz.
