Hope this helps somebody. My roommate has an iPhone 6 that began experiencing this same issue right after I bought a fancy $200 ASUS router for our home. All of our devices (between 10-20 connecting throughout the day) worked better with the new router except for this ****** Iphone.
The new router has 2 frequencies: 2.4 and 5 (there are actually two different 5gz channels for a total of three possible in this model but your router may vary)
Using the router settings, I blocked her iPhone's mac address from each 5gz channel, forcing the phone to connect ONLY to 2.4. She is back to normal and a very happy camper tonight.
Before this change, her phone forced connection to 5gz and would then proceed to drop wifi connection when moving throughout the house. Once the connection dropped, she couldn't reconnect without disabling/enabling wifi in the iPhone settings. I once worked as Verizon DSL tech support in 2005-6...No offense to my fellow geeks out there but I know nothing about apple products. I found this thread and guessed this solution after reading to page 5. Got very lucky!
My guess is software related, probably iOS update that causes this problem. Seems like many of you out there were working fine and then one day.... bam. This explains most of the issues here. Please take with a grain of salt... it might be cheaper to get a new router, especially if you just signed for a new phone. You can find many online with money-back guarantees, buy one if you can and send it back if it doesn't fix or help you troubleshoot your issues.
We have very fast Comcast internet but our plan included a cheap Technicolor modem/router combo unit. These are convenient but weak when it comes to heavy loads of traffic. We have 5 adults (2 wireless gamers, 3 telecommuters) so we aren't getting our full bandwidth without a real router. Now we also have these fancy settings to test and troubleshoot connection issues like these. Good luck out there!