I think that the problem has to do with the server configuration for the host of your WordPress site. You could probably confirm that by installing the query monitor plugin on your site. After doing that, go to the Discourse options page and refresh it. You will probably see something like this:
Click on the orange box and then look at the results under ‘HTTP Requests’. The result should look something like this (I’ve deleted my api-key from the image):
This is the result that I have been getting using the DigitalOcean 1 click install of WordPress with their letsencrypt setup for ssl.
I’d really like to figure out how to configure it so that so that the DigitalOcean 1 click WordPress install will work with https with the wp-discourse plugin. I’m not sure what the problem is.
For now, for setting up a WordPress site on DigitalOcean with https enabled, follow this guide (with the letsencrypt option.) How To Install WordPress with LAMP on Ubuntu 16.04 | DigitalOcean. This is working with both WordPress and Discourse setup on DigitalOcean with letsencrypt.
I’ll look at this some more later to try to find what the problem is that is causing the redirects. Maybe you could contact the host of your WordPress site and see if they have any ideas.