LB2120 Bridge or Router Mode?

Hi,

I see a lot of discussion about bridge mode not working.  I am, however, confused on whether I shoudl be using bridge mode or just router mode.

I have my cable modem going to the LB2120's WAN port. My Nighthawk R6700's WAN port is connected to the LB2120's LAN port. I thought you weren't supposed to have two different routers on one network, so I changed the LB2120 to "Bridge" mode. After that, everything seemed to work great -- it was working with my cable modem, and when I unplugged that modem, after a minute or so delay, the LTE internet kicked in. However, 20 minutes later, everything stopped working.  My Nighthawk R6700 showed there was no internet connection.

I couldn't get the internet working again (and couldn't even access 192.168.5.1), until I connected the LB2120 directly to my laptop.  Then I went into the LB2120's settings and changed to "Router" mode.  I then reconnected to my Nighthawk router, and after that, everything started working again.  Is my setup okay? Or should I be back in "Bridge" mode and doing something differently?  The LB2120 goes to my Nighthawk R6700, and all of the devices in my house come off of the that Nighthawk (either wired or wireless). Looking at the Nighthawk's settings page, it looks like all of my devices are being assigned an IP address by it (in a 192.168.1.X format as opposed to the LB2120 which has 192.168.5.1).  I'm not sure which is controlling my DNS though, or if I'll have any other problems down the road with my current setup. Thanks.