Extension outside LAN, phone rings but no audio on both sides

I’ve a well configured FreePbx installation. I created a new extension, to be registered from outside, I forwarded on router port SIP 5060 and RTP 10000-20000 only for request arriving from a specific IP for avoiding intrusions. I opened firewall Fail2Ban adding the external allowed IP and also on permit Asterisk field adding ExternalIP/
The phone rings correctly but when I answer no audio on both sides.
What can be the problem?
May be that I have to enable STUN? In which way? Thanks

rtp 10000-20000 set sip to pass though the router. Seems classic issues. Did you try with a soft phone?

Yes. Same problem. May be some related to router ? Where I have the asterisk server I have a netgear dgn 2200 and when I open sip port 5060 no problem but when I try to open ports between 10000 and 20000 it says that the ports are already used for other services, but its not true !!! Moreover the ports in the list are added anyway. What can I do ? Stun server has to be configured or do you think is useless ? Thanks

You did use UDP, right? These aren’t TCP ports.

ok thanks. I solved… I use UDP ports into the router. Thanks