On the iPhone.....(again, not sure if you have already done this).
I find Fixed and Rotating more of a nuisance than an asset.
Tap Settings > Wi-Fi
Tap on the blue circle "i" next to the name of the network that you are using at this time
Scroll down and make sure that the setting for Private Wi-Fi Address is Off
If you are using different names for the 2.4 GHz and 5 GHz networks, you will need to change the setting for each network separately. The same would be true if you have a Guest Network, etc.
When Private Wi-Fi Address is Off, the phone will display the IP address that has been assigned by the Wi-Fi router. It should not change if the IP address assigned by the router is not changing.