Through simple trial and error I managed to determine that this API expects you to call it like this for the /ip route:
https://weathersync.herokuapp.com/ip?73.119.54.218
Note that this is pretty unusual and would be considered incorrect by most API designers. I would expect it to be /ip/73.119.54.218 instead, but the server is not set up correctly to handle that.
Also, as TKoL mentioned in the comments, an IP like 192... will never work, because it is a local IP address that is not visible to the outside world beyond your network. If the server attempts to lookup anything based on that IP address, it will likely not find anything, or even worse it might find something about a computer inside of its datacenter, in which case it could return data that is subtly incorrect from your point of view. You can find your external IP with services like iplocation.net.
Your weather example works fine for me as-is (try clicking below in a browser):
https://weathersync.herokuapp.com/weather/28.704059,77.102490
I did not try Postman, but it may not be working for you because it does not send the same HTTP headers as a browser does. Some servers expect certain headers to be sent. You can manually configure the headers in Postman to mimic a browser, which should work if that is the case.
