I have a broadband connection. There is POE before my TP-LINK AC1200 router. Untill yesterday, everything was working fine. Today, if I disconnect the lan cable that goes from POE to router, the POE led light glows, if I plug in the lan cable back, the POE led light doesn't glows. I am unable to connect over PPPOE. I reset the router to factory default and started the wan setup again. On final step, router says something like oops...there was an issue connecting to internet. If I connect lan cable from POE in TP-LINK TL-WR841N router wan port, the POE led light glows stable.

If I setup wan connection as dynamic IP and plug a lan cable from lan port of TL-WR841N router in wan port of AC1200 router, it detects the IP address successfully.

Any hint what it may be? Any suggestions on troubleshooting the issue?