Okay, after doing some research I’ve found out that opening ports in Heroku is disabled and not allowed.
The only way around this is to use sub-domains and then in-app to use a proxy module (like subdomain-router which I use).
BUT – Heroku don’t let you create sub-domains on their domain, meaning that your-app.herokuapp.com is fixed and cannot have sub-domains.
In Heroku manuals, they demand you to have your own domain and dns provider to do such thing, by creating an A-alias (CNAME) in the dns table in your domain settings, that will refer to your app herokuapp domain, and then using the command heroku domains:add to add your domain to the allowed origin list.
You can read more here. It provides all the info you need.
Hope it helped some.