, I was able to resolve my issue with FireTV,
1) Makes sure you are in mirroring menu of fireTV stick, (Hold home button, go to mirroring option) as it won't be discoverable if you are not in that menu
2) go to Cast option on your phone
3) on right top corner, open 3 dot menu, turn off and on "Enable wireless display"
4) check list of available devices you will see FireTV there.
Possible BUG: For some reason, phone did not refresh the list automatically.