If the URLs by IP work, but the URLs by domain do not, my guess is that the DNS has not propagated yet.
How long ago did you switch hosts?
I’ve seen this recommended as a way to speed up WordPress by reducing the number of database calls. OK if the site is “permanently” in place I guess, not so much if otherwise.
The Discourse post editor can use a mix of MarkDown, some bbCode and some HTML.
* this might help Discourse Cheat Sheet
What I usually do is enclose the code block in 3 backticks
```
on their own lines. or select the code and use the </> button