Can't log in to R6400 after putting it into AP mode

To expand on the title:

I'm using a Nighthawk X6 as my primary router, connected to the modem downstairs.  Since this is in a lower corner of the house, I decided to add my old R6400 into the mix by hardwiring it to the X6 (LAN port from X6 to WAN port of R6400) and running it in AP mode.

I've successfully put the R6400 into AP mode and I can connect to the internet through it it both wirelessly and wired.  I can log into the X6 and find the IP of the 6400, and I can ping that IP address in Command Prompt from a desktop that is hardwired to it.  But if I put that address into my browser, it won't connect to anything.  I've tried unplugging the cable from the WAN port, I've ensured wifi isn't on and my computer isn't connected to a different network somehow, and still I can't log into the 6400 to make changes to it.  What am I missing / doing wrong?

Thanks in advance! 
(accidentally posted in the wrong forum; re-posting here)