Router IP Changing on its own

I have the DSL 502T modem of MTNL. One funny thing happening is if I put an ip say 192.168.100.1 for it, the next day it becomes 192.168.1.1Similarly if I use 1.1 it becomes 100.1Every evening I shut off the router, and in the morning I start it again and its IP becomes changed.I've changed the router password so many times, so I don't feel its some one changing the ip from my company.Any ideas of wats the problem?

/members/sandmax/
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Post screenshot of router page where you change its IP.

REFERENCE:Dlink Settings for Broadband

do you 'save and reboot' after changing the router address?