How do I flush the DNS cache on iOS?
Solution 1:
Turn on/off of the Airplane mode in the Settings.app or Control Center makes it fast to flush the DNS cache.
Solution 2:
I had an iPad mini that wasn't flushing DNS when I tried turning on/off Airplane mode, nor was it flushing DNS when I tried turning on/off Wifi. I had also cleared the cache and cookies, but no cigar.
What worked for me was completely rebooting the device.