R6400 router set as bridge mode keeps loosing internet on LAN ports. Am bridged to R6400v2 router.

I have two routers. The main internet router is R6400v2. I bridge a second R6400 router following instructions in manual. I have done both 2.4 and 5 ghz separately of course. The bridge works fine for a few hours but then I lose internet capability on all 4 LAN ports. I am attempting to run an amazon firestick wired ethernet to the bridged router. Any ideas or settings that I might be missing when setting up the bridge?