Hi square, the firmware loaded in your router as factory loaded firmware requires you to downgrade switch control to LEVEL 1 CTF as mentioned by Asus@SG.
My suggestion is to use your old router, get online and download the latest firmware .2678 from asus support site, or from a link in this thread which maylyn has posted.
Once u have extracted the firmware on your desktop, replace your ac87u again and then try to access the GUI at 192.168.1.1
You should be able to access the network map. Once u r there, on top of the page displays the firmware version, click it and it brings you to anither page for u to upload the new firmware you just downloaded.
Once everything is done, use the reset button on the router, press it for 5 seconds then release. Router is kinda retarded, but assured that it will reboot itself after a few seconds of releasing the button.
Now after you reset, everything goes back to default values and with the new 2678, LEVEL 2 CTF is still enabled by default but is now compatible with singtel fibre..