Hi all,
Thank you, to the kind Mod, who my message to the right place. 
I've spent most of the day trying to search for an answer to this question and half an hour on the phone with Tech support, with no success.
As the thread title indicates, the 2.4Ghz band no longer broadcasts. Tech support just drove me to frustration, as the chap kept telling me (in his best English), that there was nothing wrong with my router. He then kept repeating that I should only be using the 5Ghz band. He couldn't quite grasp that some of my devices only work on the 2.4Ghz frequency. After going round in circles and trying to keep calm, I thanked him and hung up.
I've completed three factory resets and on each occasion, the 2.4Ghz band works flawlessly, until I connect a device, then it suddenly stops broadcasting. I've tried fixing the channels, removing security (just for investigatory purposes).
As Tech support don't believe I have Wi-Fi issue, I'm hoping that others in the community might be able to help.
Thanks in advance for your feedback.