If you have a number of devices, use Private Wi-Fi addresses, see -> 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
If you use Private Wi-Fi addresses then you can assign the iPhone 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 iDevices that have already been added.
See also the following Apple articles:
Recommended settings for Wi-Fi routers and access points
Recommended settings for Wi-Fi routers and access points - Apple Support
If you need help with your Wi‑Fi password
If you need help with your Wi‑Fi password - Apple Support
Use here Private Wi-Fi addresses for 14 devices, also Android phone, tv & printer, works fine.