My issue got resolved. There was issue with the modem the service provider has provided. It has been replaced and MAC address was reset. Thanks @Kevin for your initial analysis.

My ISP is Wishnet. What is the light status of the C20 when the connection drops out? Can you get internet from it via wired cable at that time? The light status is always green in both the cases....

I have bought this TP Link dual band router on 28.07.2019. I can connect to the Internet when I initially start up the device but after 3 hours the Internet connection gets auto logged out and I...