Try first to optimize the range of the mobile network, see this Apple article:
If you see No Service or Searching on your iPhone or iPad
If you see No Service or Searching on your iPhone or iPad - Apple Support
Check Wi-Fi connection:
Connect to Wi-Fi on your iPhone, iPad, or iPod touch
Connect to Wi-Fi on your iPhone, iPad, or iPod touch - Apple Support
If your iPhone, iPad, or iPod touch won’t connect to a Wi-Fi network
If your iPhone, iPad, or iPod touch won't connect to a Wi-Fi network - Apple Support
Check the route settings, see -> 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
If they are On, turn Off Hotspot & VPN.
Contact also your ISP.
If you have multiple Apple 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.