I finally got it to work.
I tried ttnctl from home last night but it was stuck at "finding router" hence I thought the command was not working.
Today connecting my laptop on another wifi network, ttnctl uplink worked !
Here is what I did :-
ttnctl uplink 2xxxxxx 62408CD7899A72Exxxxx 2B12A66xxxxx --gateway-id mygateway
The 2xxxxx is device address
62408... is the network session key
2B12A66xxxxx is the app session key
All the 3 above values are form the console screen :- Applications->->Devices ->
Gateway ID is under the gateway tab.