I would start again.
Do a factory reset of the homehub by pressing a pin into the recess button on the rear for about 20 seconds.
Once it has re-started turn off "Smart set Up" on the Homehub.
Give the 2.4Ghz and 5Ghz frequencies different SSids. Just add a 5 to the end of the 5Ghz SSid so you know which is which. Check the 5 GHz Sync with 2.4 GHz: is set to NO.
Once you have done that "forget" the connection on your devices and then restart your devices and log onto both SSids if they support both frequencies.
I would also give the 2.4Ghz a fixed channel. I have found that Channel 13 is usually the least congested. The 5Ghz is not as congested so you could leave that on the "smart wireless".
There are a few posts about Apple devices not liking the two frequencies having the same SSid and also smart set up causing problems that do not open a browser when you first log on. ie phones and tablets.