How can we reset the device to its factory setting. Please provide me the process as i am not able to access the web interface after i change the device mode to client mode.

 You had access to the radio, changed it to client mode and now you can't reach it?

Run the Discovery Tool and see what its address is, then adjust your computer's address accordingly.

If you absolutely must reset it to its default at 192.168.2.254, open up the case, plug in your POE cable, and after it fully boots hold in the reset button on the board for 10 seconds.

i used discovery tool and it shows ip as 192.168.0.1 and when i try to used this IP in web browser it doesnot display anything as i have already changed my PC IP in same range. should i do the factory reset by opening the case.

 what is the maxium wireless coverage of DLB25 series radios

 There are too many variables involved to give you any kind of accurate answer.

It would be a good idea to start by using a link budget calculator. There are many available. Here's one

http://www.wavearena.eu/calculator/index/wifi-link-budget