Hi Giovanni,
I have tried with the change you mention, changing the domain by the public IP, but it still does not work.
Both devices are connected to the same WiFi, on a TP-Link Archer C1200 router, and in the ALG section I do not see the SIP option.
By connecting one of the devices to this router and the other to another network, the call is successful. The problem is when the two are connected to the same network (I have tested with several different networks).
It's as if FS does not find the destination number and stays waiting. Both devices are correctly registered and can be called or called by any device as long as it is not on the same network.