Problem setting up AC1750 R6400 as a bridge to an AC 1750 R6400 primary/modem router

The bridge router did not need to be connected to the modem in order to be set up.  Anyway, it looks like you got far enough to convert it into a wireless bridge, so all you need to do now is reconnect the primary router to the modem.  The bridge router should, then, connect wirelessly to the primary router.  Connect a PC or laptop to any LAN Ethernet port on the wireless bridge.  You should have Internet access.