Yet not received my service and reliving letters from TCS

Its been three months I released from TCS, but yet not received my manadatory service and reliving letters which is actually required for my next employment. When I mailed separation HR they asked to clear the IS dues to get my service letter in 3 days I paid all the dues and provided the necessary screenshot references but still no response I am tired of following up them . Could someone please help me ..where and how I should escalate the issue further. Thanks