Great progress, well done 
From your post it seems that Yunohost is running OK, but that you don’t have much use of it because of the router.
Have you already tried editing the hosts file, on the computer that you use for browsing and visiting your Yunohost (not on the Yunohost server)? That will give you the user interface (the black screen with colourful squares, instead of the white admin interface).
What router do you have? My Fritzbox has ‘DNS rebind protection’ (look at the bottom of the page).
The DNS rebind protection prevents visiting sites inside the local network by domain name. The logic behind it is: when you browse to a website with a domain name, it probably is outside your LAN.
The place to allow access to the LAN is quite hidden away, (let my try pasting a screenshot…):
I have no idea whether other routers do the same, and under which name they hide the setting.
That is when visiting your site from inside your LAN.
When you visit your site from outside the LAN, you get a router setup page. Most routers got a setting somewhere to either disable access to the settings from the outside, or an option to move it to another port. At the moment the router is kidnapping the port forwarding you made to Yunohost.