As defined in the spec, error code 62 (0x3E) is "Connection Failed to Be Established."
A good test will be to download the ble_hello_sensor app to the board and attempt a connection to the board from your iOS app. Do you see the same behavior?
If you still see the same behavior, reverse the test: try connecting your custom BLE app to iOS's LightBlue app. Does this show the same behavior?
These tests will hopefully isolate the cause of the problem to either the iOS app or the custom BLE firmware.