Have you check the router settings? -> Recommended settings for Wi-Fi routers and access points
Recommended settings for Wi-Fi routers and access points - Apple Support
Contact also your ISP, for Firmware update the router.
If it is your own router you should do the firmware update yourself.
See also -> Network Analyzer - iOS
https://apps.apple.com/us/app/network-analyzer/id562315041
If you have more 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.