How do I fix weak Wi-Fi on iPhone 15 Pro Max when other devices are fine?

By chance, do you have a VPN installed on your iPhone? What your describing sounds like what can happen when an iPhone had a VPN. VPN notoriously causes connectivity issues. They are NOT needed and don't provide the security people think they do. If you have a VPN on your phone, open Settings > General > VPN & Device Management and remove the VPN Profile and if there is an app for the VPN, remove it as well. Then restart your iPhone.

If your router is transmitting on the 2.4Ghz band, it is most likely your router and the solution is to create a 5Ghz only network on the router and connect your phone to it. When your router is broadcasting multiple bands, it will connect to the first one it sees and usually that is the 2.4Ghz band since it has a longer range, but that band has serious problems when multiple devices are connected to it. You may certainly be 5-7 feet away, but the interference on the band and the channel overlap is always going to give you a poor connection. I understand you do not think the router is the problem because other devices are working, but you fail to realize that there is a limit on how many devices that can maintain a reliable connection on the 2.4Ghz network. Change the router to a 5Ghz only WiFi network and compare your results after you connect to that network.

While it is possible there is a hardware problem with your WiFi antenna, it is not likely. Without support available for a repair option, I would recommend to follow my previous suggestion as it costs you nothing to try.

You Could Try Factory Reseting It! Most Of The Issues I Have Seen Have Been Not Being Set Up Properly By People! If this does not work contact apple support with your issue or request an trade-in for the same phone!

Yours Kindly,

Ethan-IT