I was using TP LINK Wireless N ADSL2+ Modem Router which was one side connected to landline phone and another side had 4 ethernet port and also act as wifi router. Now I brought BSNL WiMax where the internet comes directly in RJ45 cable. I don't like to buy a separate wifi router. Is it possible connect this RJ45 cable to one port out of 4 port of that existing ADSL2+ Modem Router to get wifi connection and also to connect desktop system with balance 3 port? Can anyone help me in this?
A few ADSL modem/routers have the capability of separating the modem section from the router section, and offer a WAN port (usually by re-purposing LAN port #1). But the vast majority of ADSL modem/routers hard-wire the WAN-side of the router to the downstream-side of the ADSL modem. The TP-Link unit that you have seems to be of the latter hard-wired type, and therefore unusable in your new setup as a router.
If you disable the router features in the TP-Link (e.g. DHCP server, NAT, firewall etc.), then the LAN ports could be used as a switch and the wireless section used as an access point. Such a setup requires that the BSNL WiMax unit be the router for your network, and provide DHCP and NAT services. As you planned, one of the TP-Link's LAN ports would be connected to the BSNL's ports.
There are three interfaces that you have to setup in the TP-Link: Internet, LAN and Wireless.