Can you get any other REST queries to work? If not, then first make sure you have authentication set-up correctly and there is nothing limiting the objects you are trying to access. You would check in the Settings > Integration > REST
Second, make sure you made the appropriate server and web.config changes
My suspicion is there is something not configured correctly for the REST service, but if you can do other REST queries successfully and just not these ones, let us know and we can add some additional thoughts.