Hi Murali,
Could you please compare the schematics of the USB dongle to the connections you're using for your TTL converter to see if they're connected correctly? The problem could be related to pin connections.
The schematics for the USB dongle can be found in this page of our website as "JN-AN-6039", and the schematic for the JN5168 USB dongle islocated in the following path:
<…\JN-RD-6039\JN-RD-6039-JN516x-USB-Dongle\DR1198_OM15020_JN516x_USB_Dongle_1V1\Schematic\DR1198_Schematic_1V0.pdf>
Please let me know if you still have issues.
Best regards,
Sebastian