@MithinXavier 

May I know the model number of the main/root router? Is it also TP-Link?

How about those devices connected to the main router, will it connect automatically when leaving and getting back? 

Ensure the firmware on both the main and the C20 are up-to-date.

Please confirm if you can still get a correct IP address from the network when getting back. If yes, try to ping the gateway, and show us the ping results.

@Kevin_Z 

Thanks for your valuable reply,

The problem is i don't have any physical access to the main AP since i took this connection on sharing basis from my neighbor and he give me the password to access WIFI. But from the MAC address which i got from the network analysis brand is something like DWnet Technologies(Suzhou) Corporation (7C:8F:DE:2D:D3:00). I had tried connecting my mobile devices with main router WIFI which is working perfectly without any problem.

Im not aware how to get the ip address and ping the gateway from mobile. Please advise so that i can share that result also.

@MithinXavier 

I see. You can check this guide below to find the IP address on the mobile when it is connected to the C20 network while there is no internet:

https://www.makeuseof.com/tag/find-ip-address-mobile-smartphone/

How to ping on an Android, check this:

https://itstillworks.com/ping-website-android-phone-34426.html

You can search online for the instructions if yours is an iPhone.

Back to the no internet access issue when coming back home, I think you can easily reproduce the issue by disabling the Wi-Fi connection on the phone then re-enabling it, please give that a try, check if the issue persists every time when disabling then enabling.