Thank you very much for your help. It works fine now !
Your recommandation : “try creating the network with your ip for internet” is very precious.
We can login to a QNAP NAS either with the local IP 192.168… … or with the internet IP 82.64 … ….
The solution is to login with the internet then to install phpmyadmin and then to intall WordPress and build the network for Multisite.
My mistake was to make the installation with the local IP.
