Unable to set external_signaling_port in freepbx

Hello,
I’m new to freepbx.
I’m trying to set a NAT configuration for pjsip and chan_sip using non standard port.

The only way to be able to make it work was to explicit a port for externip (like: externip=189.96.211.82:50060) and external_signaling_port=5321.

Unfortunately I’m unable to find any way to set external_signaling_port in freepbx. If I add it to pjsip.transports.conf (under [0.0.0.0-udp]) manually it is deleted every time I click “apply” in the GUI.

Any idea on how to do it?

Thanks
Fabrizio