Behind a NAT, you need to do some portforwarding. Reading your answer I see you have some knowledge in network administration so I think you know what to do. 
You can try with installing an SSH server on yunohost on the port you want (666 for sample), do the portforwarding/route on your router and try to reach yunohost via SSH. After, you can try to reach your server via 443 port.
You can also try to configure a VPN server on yunohost via Openvpn.