We are working with a C35 and a V2110 wireless controllers runing the firmware version 9.21.20.0001. Since the release of IOS 11.4.1 (july, 16, 2018) our APs (3705i) are, randomly, dropping most of the IOS devices.
It looks like many IOS 11.4.1 users are experiencing WiFi issues after upgrading to that version. Some users (see links and threads below) are indicating that they have been able to back rev to resolve this issue ("Hello, just to verify that I to updated to 11.4.1 and was faced with exactly the same WiFi problem. After trying advisory fixes which didn’t work (e.g. clearing network settings) I tried a re-install of the update via iTunes but to no avail. Finally I have now downgraded back to iOS 11.4. On iTunes Which has now fixed the problem.").