Guys, THANK YOU TO EVERYBODY who took time and answered my silly questions.
Here is the solution:
1. I login in to my Router web interface using
http://192.168.1.1 and changed the LAN ip to 192.168.2.1 from 192.168.1.1 (my modem already has 192.168.1.1 as its LAN ip)
2. And I changed the 'Start IP' to 192.168.2.100 from 192.168.1.100
'End IP' to 192.168.2.199 from 192.168.1.199.
From now on you have to use
http://192.168.2.1 to login router web interface.
Viola. Router interface working!

thanks!