Configure Asterisk for NAT external devices

General information for the app: Troubleshooting
Description of requirements/features: My Incredible PBX, FreePBX was working fine with my Netgear Router WNDR3700v4 and also WNDR3400v3, with several internal devices and several external devices. Port forwarding used to be from port 5000 to 31000. Everything was going well for internal as well as external devices.

Then I flashed the WNDR3700v4 firmware to DD-WRT. I set up the same port forwarding. Everything works well with internal devices. External devices are also able to register and make calls. There is however just one problem: there is no audio (both directions) in these calls from external devices. When the external devices come home and connect through WiFi, then audio is fine.

What could have been messed up due to the firmware change?

PS: I also tried reverting to the Netgear firmware, with no other change in the network layout. Audio starts working with external devices with the Netgear firmware, and stops working with the DD-WRT firmware.

And now, even the WNDR3400v3 is showing the same symptoms even with the factory firmware. The WNDR3700v4 is still working fine, but only with the facotry firmware.

PS: With each change of firmware, I was allocated a new public IP address and I have taken the care to update it each time in the SIP Settings of the PBX, before doing the audio test.
Extra notes: