Read this if your iPhone or other iOS device is dropping wifi frequently. This and many more good tips are on the Apple website.
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.
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.