HELLO
I have similar problem me too, can you tell us how did you solve your issue.
My a Dlink DSL 2750U FRMWR 1.09ME, has problem authenticating to my ISP (red internet light). this usually happen when I press the "Disconnect" button in internet setup section of the router and then "connect", the router connects back to internet but after a couple of hours/minutes, the router gets disconnected and can not authenticate anymore, this problem happens for the second time, I have another spare router that I tested, it authenticate properly and I can access internet without problem. I noticed that this problem occurs only when I press that "Disconnect" button in "Internet Setup" section.
I tried re-setting authentication credentials many times but still not want to connect.
I also tried resetting the whole router setting and reconfigure it again, but still doesn't want to connect
Location Morocco