How do I save a manual HTTP Proxy setting?

I know how to configure a manual proxy on iPhone (settings->wifi->network_i_want->http proxy->configure proxy->Manual).

However, when I enter the proxy server and port (authentication is *not* required nor enabled on the proxy), and hit Save, the phone simply takes me back one page, but the HTTP Proxy "Configure Proxy" is still Off, and none of the data is saved under the proxy.

And, of course, the traffic is not passing through the proxy service.

I've tried this on a couple of different wi-fi networks, both 2.4 and 5 GHz, doesn't save.

I'm on an XS Max, version 12.0.1 .

How do I get this configuration to save?