Read this if your iPhone or other iOS device is dropping wifi frequently. This and many more good tips are on the Apple website.
[pullquote]iOS 8 and later uses a randomized MAC address when running Wi-Fi scans. The scans are conducted when a device isn’t associated with a Wi-Fi network and its processor is asleep.[/pullquote]
We’ve been having this problem for approximately forever. We had been incorrectly blaming our Sonicwall TZ300W security router for this problem. It turns out that Apple made a “small” change to iOS that caused most (if not all) of the problems.
Short version: Make sure your router is broadcasting its SSID. But this was not our problem. Further down the page we see this:
My paranoia led me to implement MAC filtering on both our routers. Turning MACoff helped. A lot.
There’s a lot more good advice on the website linked above. Read it and bookmark the page.