Just putting this out in case somebody makes same mistake as I did -
Inside NAS (Synology DS211j), I tried configuring it to be connected to Internet via PPPoE. Result failed.
Now, this resulted to me losing or having very intermittent internet connection. Connection only became good when NAS is disconnected from LAN. Upon spending about 2 hours looking for solution, I stumbled upon network setting inside NAS. There I saw PPPoE setting was activated and connected! So I disconnected it and everything is now fine.
Whew, I believe I rebooted the modem router about 4-5 times!