Update on the case:
i saw that OpenVPN internal settings were set as "TCP" instead ad "UDP". Thus, OpenVPN cannot connect.
This should not be a problem, since I was trying to ping/access WebUI from WAN and from Mobile, failing both, not from OpenVPN.
However, after setting correctly "TCP", beside correct connection of OpenVPN, also WAN started responding to PING and http access!
By now only access from mobile doesn't work (note again: mobile hasn't public IP but mobile provider gave me access to their VPN. i'm contacting them to see if all is ok)