We set up our staging environment for ArcGIS Enterprise 10.6.1. Everything works fine when webmaps, apps, and etc. are accessed internally; however, we are not able to view any of the layers in the webmap when accessed outside of our network using the ArcGIS field apps(collector and Explorer). We get the error "a server with the specified hostname could not be found".
We have a multi-machine deployment with the VMs for the datastore and Server for ArcGIS and web adaptor inside our network. The VM with Portal for ArcGIS and the web adaptor is in the DMZ... image of setup is attached.
What is wrong or missing in this setup?