I don't think it's poppycock at all to need to sometimes disable (or not use) 5GHz to set up a 2.4GHz device!
When you have both the 5GHz and 2.4GHz using the same SSID and password (which is a common, easy and useful way to set devices up to also seamlessly switch between the two frequencies depending on strongest/nearest signal etc), it's a common problem for some devices to require 5GHz to be unavailable (for some strange reason) while setting up that 2.4GHz device as the device's phone/tablet App communicates as part of the new device's initial setup.
Sometimes setting up a new device while the 5GHz is active will work because the phone/tablet running the App is currently using just the 2.4GHz network (e.g. out of range of the 5GHz network), but when it's using 5GHz, then the App can't complete the initial setup but should/may work fine with 5GHz back on after setup completion.
It's not always easy to tell what frequency your phone/tablet is currently using but because 5GHz doesn't travel as far as 2.4GHz, in theory it may help to move further away from your Wi-Fi base station so your phone/tablet switches to 2.4GHz - or just turn 5GHz off in the router/base-station, or perhaps most easily temporarily change the 5GHz SSID or password (in the router) so that your phone/tablet can't connect to 5GHz while you set things up.