I have recently upgraded my Broadband package and this came with a HG633 router, blog on this is here: Router Upgrade, a day or so after installation Apple updated my daughters iPhone 6 after this update the phone kept dropping in and out of WiFi connectivity.
A the router was new and my Samsung Galaxy worked fine as did the i tablet mini, so it pointed to her phone.
Cutting a long story short including swapping the phone for a new one, the problem seems to stem from the fact that the HG633 has two WiFi frequencies available (5GHz and 2.4GHz), both being ON by default and sharing the same SSID, this makes the iPhone 6 very unhappy.
The problem of frequent disconnects and reconnects was solved by entering the router setup and renaming the WiFi SSID’s so that each frequency had a unique identification, based on the signal strength is the frequency you ‘pair’ with, this has cured the problem.
Hope this helps bring peace and harmony back into the homestead.