I have a number of devices connected via wifi: a mix of Windows laptops, android phones and an Apple iPad Pro.

I had noticed recently that most of my devices were using the 2.4GHz band, not the faster 5GHz. After repeatedly trying and failing to connect to 5GHz with each device, I jumped into my Archer's admin interface to see what was up.

I discovered that the only device that was connected was my iPad Pro. All other devices failed to connect to 5GHz, but were perfectly happy connecting to 2.4. 

After turning off the iPad, all other devices were suddenly able to connect to 5GHz. Restarting the iPad allowed it to also connect to 5GHz. I can now also disconnect and reconnect to 5GHz on all devices.

Very strange, but I'm not going to waste any more of my time trying to find out why.