Since you couldn’t get a connection even before HTTPS was enabled on your WordPress site, I’m not sure what the problem is. I can look at this some more later.
Possibly, looking at the access log for your forum can give some information about what’s going on. To get there, ssh into your forum’s server, then:
$ cd /var/discourse
$ ./launcher enter app
$ cd /var/log/nginx
$ tail -f access.log
On your WordPress site, click the save options button on the WP Discourse Connection tab. You should see some sort of response in the Discourse access log.