Your problem is very common or may be very uncommon but the major matter is that nobody knows how to solve it except your ISP. Contacting your ISP will first doubt and may say server down is the reason . Error 651 is major and it says modem has reported an error. Actually what you basically need to do is:
1. Go to your ISP and tell them to delete all old configurations and create new one with DYNAMIC IP and connection to be set as ALWAYS ON. ( please dont experiment yourself and dont follow websites or internet for configuring your BSNL modem)
2. Set up your modem perfectly with strong tight and secured wires and connection to your pc and connect the link wire with the modem.
3. When new configuration is done and modem is set, switch ON the modem & call them and tell them to RESET your link. ( this is very important or you cant connect and this step will also reset your password too)
4. After reset , connect to your internet.
THE MAIN POINT IS THAT, YOU CAN CONNECT THE
ROUTER TO MODEM AND USE BUT ONLY WHEN YOU HAVE HIGH BANDWIDTH OR HIGH SPEED INTERNET. DO NOT CONNECT ROUTER WHEN YOU ARE USING LOW SPEED INTERNET. MINIMUM 2 MBPS SPEED IS RECOMMENDED FOR ROUTER. DON'T USE ROUTER FOR 512 MBPS CONNECTIONS OR YOU WILL GET BACK TO THE PROBLEM AGAIN AFTER 2 DAYS, 1 WEEK OR MAY BE 3 WEEKS.
CHEERS AND ENJOY NETTING
thanx a lot to all who helped 
