Port Forwarding - Error Adding Entry

Encountered the error message: "Error Adding Entry" when trying to add an entry under Port Forwarding. Not sure what I'm missing here. Did search on the forum, found 2 similar cases but didn't see how it was resolved. And key difference being my scenario can't even add an entry.

Any help is much appreciated. Thanks!

NightHawk M1

Hardware Version: 1.0

Firmware/Bootloader Version: NTG9X50C_12.06.12.00

Firmware Build Date: 2020/04/26

Using TPG Singapore 4G/LTE SIM. 

IP Passthrough: ON

DHCP: OFF

Ethernet Standby: Disabled

DMZ: Enabled