Issues with BSNL Broadband or my Modem?

After issues with my old modem getting error 678 etc.

I got a new modem router. TP-LINK TD-W8961N 300Mbps

I set up the new modem router using PPPoE mode entered username password, set up the wifi and it was working fine since yesterday till this afternoon.

The internet stopped working I opened the modem setting page and the modem had gone from PPPoE mode to Bridge mode automatically. I changed it back to PPPoE but the Internet is still isn't working. What is up with this BSNL Broadband?

How can I solve the issue?

So I reset my router modem and again, set it up as new now the internet is working. Does this mean I have to reset it every time the internet stops working?

I am not able to understand the issue.