Some things to try:
Make sure that the "smart set up" is turned off on the Homehub. See link how to do that.
Try giving 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 and then restart your devices and log onto both.
You'll need to connect your other devices that can use both frequencies to both SSids if you want them to be able to use both frequencies.
See this link
You could download and install inSSIDer which is a program that will scan for and show wireless networks.
See link for inSSIDer
This will show you the wireless channels that your Homehub/router is transmitting on as well as the surrounding networks. You want to take a note of the channels in use and change your Homehub onto a channel that is not being used.
See Link how to do that.