If your add on domain has not been set up to use nameservers, you will need to create a CNAME or A record in your DNS control panel (where you bought your domain name) to point your en subdomain to your server. How you do this varies between companies, but usually look something like this:

Here you can see that the test and www subdomains both point to the main domain. If your subdomain will be on the same server as your main domain, your en CNAME record can be set up the same way as this example to point to your main domain.
This is required because setting up a subdomain when you're managing your own DNS records requires two things:
- A DNS record for your subdomain so that the DNS server knows which IP address to send visitors requesting
subdomain.example.com
- A server record for your subdomain so that the server knows which folder and files to serve when a visitor requests that subdomain.
It sounds like you have set up 2, but not 1.