Getting error "Unable to tunnel through proxy." from Omnistudio Http Callout

When calling the Weather API provided in a trailhead  "api.weatherbit.io" we are getting below error:

"Unable to tunnel through proxy. Proxy returns \"HTTP/1.1 503 Service Unavailable\""

We added "api.weatherbit.io" in the Remote Site Settings but the error still persists.

We found here that the issue might be because the Firewall at the external system blocks the calls made from Salesforce. Thus, we tried calling a separate API from Omni studio and we are getting the response. 

Has anyone faced this issue while doing the trailhead "Extract External Data with an Integration Procedure". 

Any help would be appreciated. Thanks.