If you have successfully installed YunoHost and passed through the post-installation, you probably have a working server.
In a web browser, access to your server via the domain name you just entered at the post-installation step.
For example: http://mydomain.com
Go to https://www.whatsmydns.net/ , enter your domain name in the field and click Search.
If you do not see your IP address, or if there are red crosses everywhere, then you have probably misconfigured your DNS.
If your DNS is properly configured, and your server is accessible locally, you may have network ports blocked or it may not be forwarded by your router.
In order to verify this, try accessing your server with a client outside your local network. For example via another WiFi access point or with your mobile phone in 3G/4G.
If the server is unreachable from outside your local network too, then the problem probably comes from your router's configuration.
You can also redirect ports manually to your server's local IP address on the router's configuration interface.
If the server is accessible from outside your local network, but unreachable with its domain name on the local network, then your router probably lacks hairpinning.
Here is a tutorial to access your server on a local network and bypass the hairpinning problem. The tutorial proposes a first solution to create a redirection with DNS of the ISP box and a second solution to modify the hosts file of the clients to instruct it to access the server via its local IP address. The first solution is preferable because it's not necessary to modify the hosts files on every computer on your local network, if you are using many different clients.