When the workflow calls the api via the xtension, the call is likely coming from your NAC tenant to the web service.

If the web service requires the whitelisting of the caller’s ip addresses, you may have to whitelist the following NAC ip addresses base on the region of your NAC tenant:

https://help.nintex.com/en-US/platform/AllowlistIPAddresses.htm

What API are  you trying to connect to?  Is it internal or external?  If internal, then you have to install and go through the Nintex Gateway.  If external, then are there any security restrictions on that API?  Have you tried a different external API and gotten it to work?  Try the AccuWeather API, which is a simple one to get a response, although you may first need to register with AccuWeather and get an API key.

Use “Call a web service” action.

Add this URL,

https://dataservice.accuweather.com/currentconditions/v1/351198?apikey=

Then add the API key.

The Request Type is “HTTP Get.”

Under “Response content,” add a variable, which will be Text (it will be JSON text).  If you get a response without error, then you know that you can successfully call an external web API, which then tells you that you have to figure out why your other API is not allowing Nintex to connect.  Maybe it requires an API key also.