I just got AC86U from oversea, had it setting right with my unifi ID and password, Enable PPP echo detect, with Interval 6 and Max failures 10, MTU and MRU same as 1492, everything else in default settings.

Updated to latest firmware, 3.0.0.4.384_21045

Reset both main modem and AC86U multiple times....

But still no internet access as the connection LED light remains RED.

So i pop back old D-link TM free router, it connected internet perfectly!

So what goes wrong here Anyone may guide me here, many thanks.

So it's just purely firmware issue? Wondering anyone encountered this before...

Thanks, have it solved. The ISP profile is a hint why it won't work as the router light back to WHITE and Internet status stated CONNECTED, yet still no internet access at all due to a TM message display the password is invalid. I contacted TM twice to reconfirm my login info and they said nothing is changed and everything looks "ok". So i pop back the good-old unifi free TP-link router to check again the UNIFI password stored inside (must updated the firmware to enable the password review option), and voila, it wasn't the same one as TM provided to me. Weird, huh?

I entered the new Unifi password into Asus router and everything goes back to normal. So its not the firmware, not the settings, and what struggling me for days is TM passwords. (I still have no idea how this new passwords come from?)

Thanks everyone for help. By the way, since i have 2 units of AC86U, the merlin firmware temporarily not supported Aimesh so i switched back the factory firmware.