Connection refused when adding ApplicationIntent
Ok, got it. I had to force the TCP1433 port in SQL Configuration manager... That was only that.
I found it when trying to connect with a wrong password : SQLServer denied connection because of this wrong pass.
So, the mistake was between the listener and the server supposed to accept the connection... which was the second one, because of the read-only intent.

And this second one, even if it was listening on TCP1433, wasn't responding without explicitely configuring it in Configuration Manager.

Sorry!