Unable to access new RB5009UPr+S+IN router

Yes, that more for getting into netinstall mode.

Also, I would remove all cables other than power and the one you are connecting to the RB5009. It almost seems there may be an ip conflct. Does your network have any other devices in the 192.168.88.0/24 network?

Some relevant posts that corroborate what jaclas said: