Please read the manual as everything is well explained: http://www.tp-link.com/Resources/document/TL-WR841N_ND_V8_QIG_7106503891.pdf

The settings are simple to configure the TP-Link TL-WR841N for your Reliance Wired Broadband using the ZTE ZXDSL 831.
Connect the ethernet cable that comes from the ZTE modem to the WAN port of your TL-WR841N.
Make sure the power and the phone line is connected to your ZTE modem and powered on.
Power on the TL-WR841N and connect another ethernet cable to the port 1 and connect the other end to your PC/Laptop.
Set your NIC settings to automatically obtain IP address.
Open your favourite web browser.
Type in 192.168.0.1 in your address bar.
Login dialog box will appear.
Type the following.
Username: admin
Password: admin

Click on Network-> WAN on the left pane.
Enter or select the following settings
WAN Connection Type: Dynamic IP
Then click Save

Then click on Wireless->Wireless Settings
Wireless Network Name: "Enter your desired name"
Region: India
Then click Save

Then click on Wireless->Wireless Security
WPA/WPA2 - Personal (Recommended)
Version: WPA2-PSK
Encryption: AES
PSK Password: "Enter your desired password"
Then click on Save

Then click on System Tools->Reboot
Click on "Reboot button"

Wait for a couple of minutes till the unit to reboot and startup.

After reboot.
Try typing "http://www.reliancebroadband.co.in" in your address bar of your browser.
You will get to see the reliance login page of your Internet Connection.
Enter the credentials.

After successful login, try connecting any of your wireless devices to your Wifi network and try accessing your internet, you should now be able to access the internet :)

If you face any issues in this regard, please do reply.