Did you rebuild after you changed the value? (Enable a CDN for your Discourse, which is linked to in the yml file you edited says to rebuild after the change.)
No, what needs to be done is to do a rebake of the posts to fix the references to the old CDN. See Disable CDN causes old images with CDN URL can not be shown for two ways to do it. The simpler way is:
# ssh into server
cd /var/discourse
./launcher enter app
rake posts:rebake