I ask the same Question in some other Forum
https://www.musiker-board.de/threads/marshall-code-blootooth-problem.657279/#post-8205236
And there is Help.
My Rig:
Amp Firmware 1.3
Amp DSP 1.1
Amp Blootooth 3.1
Gateway Version 1.4.1
iPhone 5s iOS 10.2.1
Here is the Answer
If you have activated Bluetooth on the iphone before you switch the code back on, then the amplifier should automatically connect to the iphone. You can not press the Bluetooth again on the amp. Otherwise there is the problem, which you described. So once manually couple and then it should go automatically. With me it was so so ....