I have an issue with opening the approval task from external network. The Nintex OOTB task is opening fine when I tried to open in our corporate network. However, when I tried the same task to open from the internet, below message is shown. I am able to access to our portal. 

Forbidden

You don't have permission to access /_layouts/15/NintexWorkflow/ApproveReject.aspx on this server.

Does anyone have the same experienced? We are using Nintex Form and Workflow 2016 for SharePoint 2016 On-Prem. Thank you in advanced for your sharing.