iPhone 6 Proxy is not working

Greetings,

I have an Iphone 6 with IOS 12.5.7.

I want to use an Proxy, which I configure through the settings in WiFi (Server, Port, Username & Password). Using the Proxy on my Iphone 14 works without any problems. But on the Iphone 6 it won't connect.

Does somebody know what do to here? Does the Iphone 6 or the IOS version not support something?