I tried upgradeing to 4.11, but ever since the upgrade I can no longer access the router.
I tried useing the Neighbour Viewer but that didn't work either. Any ideas on what I can do to get back into the box? It's a RB750G.

Cheers
Brad

Those unfortunately don't have serial ports. NetInstall via PXE boot and restore to factory defaults, and also at the same time re-apply the OS version you were trying to upgrade to. You will lose all settings.