Now at my mother’s place, and recently on another private wifi, the problem appears.
Thanks for your explanation.
Guess that at your mother's place Private Wi-Fi addresses is use.
You can assign your iPhone a MAC & a Private IP address.
Log in to the router, add your iPhone NAME + iPhone MAC + 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 the following Apple articles:
Use private Wi-Fi addresses on iPhone, iPad, iPod touch, and Apple Watch
https://support.apple.com/en-us/HT202068
Recommended settings for Wi-Fi routers and access points
https://support.apple.com/en-us/HT202068
Erratum: iphone 6s, not 7
No problem, my iPhone 4S is added too in my router and you can add any other devices such as a printer, tv, Android smartphone.