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 is red crosses everywhere, then you have probably misconfigured your DNS.
If your DNS is well configured, and your server looks is accessible locally, you may have network ports blocked or not forwarded by your router.
In order to verify that, 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 this 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 of hairpinning.
Here is a tutorial to access to your server on a local network and bypass hairpinning problem. The tutorial propose in first solution to create a redirection with DNS of the ISP box and on second solution to modify hosts file of clients to say to access the server via it's local IP address. First solution is preferable because it's not needed to modify hosts files on every computer of the local network, if using many clients.