Hi @hemantchau11549
Try using the laptop WiFi as close as possible to your network router i.e. <20 cm and check if the signal is stronger and also that the connection is stable.
If it is there may be an antenna problem with the WLAN adapter in the laptop.
Here's the [https://dl.dell.com/content/manual13059505-inspiron-15-3511-service-manual.pdf?language=en-us|servioce manual] for the laptop.
Go to p.27 to view the procedure to remove the WLAN card. You don't have to remove it but this will allow you to check that both of the antenna cables connected to the card are securely connected or not.