On the iPhone, the proxy is in the Wi-Fi network settings, under configure proxy, in manual mode. The server is the IP, the port is 20001 for HTTP, and authentication asks for the username and password.
What this configuration does not do
It applies to the configured Wi-Fi, not to the mobile network. And it does not reach an app that ignores the system proxy. So test with the site open in the phone browser before counting on it.
SOCKS5 on port 20481 does not fit in this setting, which only accepts HTTP.