See 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 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 (and other devices) a MAC & a private IP address.
Log in to the router, add your iPhone NAME + iPhone MAC addres + 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.