Unfortunately, you have not done the necessary thing but something else.
I have a Help page at:
http://greghelp.991552.n3.nabble.com/Embedded-Forums-and-https-td4012846.htmlwhich may assist you. Make sure you have worked through all the sections before 2.3 where you enter your sub-domain at Step 2 on the screenshot there.
Currently, there is no evidence yet that you have embedded your forum in your blog. Had it been embedded correctly and https not been implemented by Nabble you would see a simple link to your forum. If Nabble had implemented https it would display in full as my examples do in their host pages.
When you change the domain name correctly it should show your Nabble forum as this example does:
https://forum.stalhambaptist.org.uk/(Note how the "Bulletin" sub-forum is embed in a page on the main site, but when you use the "SBC Forum" link you return to the unembedded forum.)
Here is another example where a forum is embedded in the page at:
https://www.seahawk17.org.uk/forum.phpand the embedded forum is assigned to the sub-domain:
https://forum.seahawk17.org.uk/As you see from the screenshot below the forum is "redirected" meaning that unlike the example above it is not possible to visit the forum without it displaying within the page within which it is embedded.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.