TL-WA850RE is a new release and would take few months to come to India.. However you can deploy an access point working on a repeater mode to amplify your signals ... Luckily Kolkata has a TP-Link...

You can make stable connections within 250 meters even in NLOS conditions and pre installed antenna ... Give it some height ..

15 Kms means Clear Line Of Sight ... There should be nothing in between two connecting routers ... No building .. No Trees .. Give it some height .. At client end use a 24dBi Grid antenna ..

This seems to be a case of interference .. Survey and check wireless networks available .. Use the channel which is free .. Also first test it in indoor mode of distance settings

This will probably solve your problem ... Happened to me .. Go To SNMP and activate it ... add permissions and reboot .. Worked for me

You should try the range settings .. Wireless > Distance Settings > Select Manual and set distance range to 110 % Approximately Also Wireless > Wireless Advanced > Antenna Settings : Vertical/...