I think your problem is Hairpinning
So, I think the easiest way is to have a local DNS. To match your domain name to the private address of your Yunohost.
Maybe you can first test by filling this correspondence in the hosts file of your machine.
For windows, the file is : C:\Windows\System32\drivers\etc\hosts
and linux is /etc/hosts.
It must be modified as an administrator.
You can see this subject for exemple.
There are other, but they are only in French …