Hi Nurice,
If one of the problems you’re encountering is broken images, you probably need to update Hotlink Protection by adding https://example.com/directory and https://directory.example.com to the Enabled list. (From cPanel, click the Hotlink Protection icon.)
I have a similar setup to you — I have my main domain https://www.maindomain.com, and my WP blog as a subdomain at sub.maindomain.com, which is actually an addon domain https://www.domain2.com.
In Hotlink Protection, I’ve added https://www.maindomain.com, sub.maindomain.com, https://www.maindomain.com/sub, and https://www.domain2.com to the Enabled list.
On the WP General Options page, I use https://www.domain2.com for both the WordPress address and the Blog address and it works fine.
Probably more information than you needed, but if you’re encountering the broken images problem, it might save your sanity. ??