You can use Private Wi-Fi addresses for all your devices, if you use Private Wi-Fi addresses all your devices are protected. You don't give any other devices access to your Wi-Fi network.
If you use Private Wi-Fi addresses then you can assign the iPhone (and any other devices) a MAC address & a Private IP address.
Log in to the router, add your iPhone NAME + iPhone MAC address + private IP address.
Your iPhone MAC address is: go to Settings -> General -> About -> the Wi-Fi address (note this address).
Assign a private IP address to the iPhone, for example 192.168.x.xxx or 169.254.x.xxx.
This depends on the router & other iDevices that have already been added.
What is important, check the list and the last xxx must be different from the devices that have already been added.
Here all devices (Mac, Windows, iPhone, printer, TV and others) have been added in my router that way for years, and it works well.
See the following Apple articles:
Use private Wi-Fi addresses on iPhone, iPad, iPod touch, and Apple Watch
Use private Wi-Fi addresses on iPhone, iPad, iPod touch, and Apple Watch - Apple Support
Recommended settings for Wi-Fi routers and access points
Recommended settings for Wi-Fi routers and access points - Apple Support