This article covers turning off MAC randomization on iPhones.
Step 1: Open your settings app and go to Wi-Fi. From here, press the blue 'i' next to the network you are connecting to.
Step 2: On the next screen, set the "Private Address" switch to off.
Step 3: A pop-up message will appear prompting you to rejoin the network, press rejoin and you will be reconnected.
That's all!