Where does this 3 month figure come from? Unless you've been told otherwise by BT, the moment your new service starts, you're locked in for the duration of the contract . There are heavy exit fees if you want to cancel or downgrade early.
Apple devices seem to have a problem with the dual frequencies having the same SSid.
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 on your devices and then restart them and log onto both.
See this link
Also make sure you have turned off "Smart Set Up" on the Homehub. See link how to do that.
Here's a speedtest I just did on my iPhone on 5GHz on my HH5