I am still having fundamental problems following the guide for instagram setup.  You can see from this image below that it doesn't match in many ways the image in your own guide at step 3.

The latest problem I have is that there is no "Instagram Testers" area of the roles page.  This is visible in my screenshot.

Your guide is a year old now and it seems to me that it must simply be out of date and that Facebook likely changed many things. 

Can i please ask you to go through the process at your end as I'm sure there must be something wrong that isn't just at my end.   

Thank you we've gotten further.   We have created a new app and that offers us the "instagram basic display" product, which allowed us to follow all the instructions.

However, it still isn't working.   When trying to login as a user in the front-end interface of wpdiscuz, after being sent to instagram and logging in successfully we are returned to a white web page that just states:

{"error_type": "OAuthException", "code": 400, "error_message": "Insufficient developer role"}

I have the same problem.
From the tester's account, I can log in. And just with the other no, it's a mistake.
It turns out that users cannot write messages.