Yes, as you mentioned, http://myserver.local:22300/api/sessions won't give anything else than an error.
But beeing how it is (the server is perfectly accessible through a webbrowser and even Win10), I guess the problem is at IOS and MacOS...as you said...
I can't imagine a ssl-connection is required, however I'm not able to test it, since I don't own such a device.
So someone else has to help, sorry.
Edit: Just to make sure, the IOS/MacOS Devices have access to the server, did you try the webbrowser-thing also on them?