You can only have one device connected to SBG8300 that’s in bridged mode and the Eero router will need to be reboot so that it gets a public IP address from the ISP.

I have similar scenario and stumped. Senior citizen here. Thanks for helping.  New service Xfinity w/new SGB8300 and Eero mesh system.  Do I need to put both the SBG8300 and Eero in bridge mode?  I was having dropped signal to TV and no internet connection for minimal seconds.  All my devices wifi network had previously been Eero wifi log in.   I called Eero and tech had me put eero into bridge mode and then said all my devices need to connect to the SBG8300 log in.  No mention of putting SBG8300 into bridge mode.  I do not think this set up is correct because devices at further distance from sbg8300 are showing week signal and the eero wifi is better strong signal.
 

How should both devices be set up?   I am thinking sbg8300 in bridge mode and then plug in eero with eero bridge mode turned off   All devices would then only need to connect to eero wifi   My devices include TVs, iPhones, iPads, and RING cameras     Thank you!! 

its either/or. You can’t have them both. 

If the SBG8300 is in bridge (modem only) mode, its simply a modem. It can only have one device connected to it (the eero router). 

If the eero is in bridge mode while the sbg is as well, then nothing is managing dhcp/nat and devices don’t get an ip address. So one or the other. 

Thanks for the quick reply.  Follow up question(s):

Which one should I have in bridge mode?  Does it matter?  
and once it is determined which one should be in bridge mode - which wifi would I set up to use for my in-home network - the bridged device or ?

Previously when I had a leased Xfinity combo with the eero attached the eero was not set in bridge and was the wifi on my network for all devices to properly access.  I am assuming right now the sbg8300 is not set to bridge mode nor do I know how to verify that in the sbg8300 settings   TIA 

Update:  I think I have everything properly set up now. Arris SBG8300 is working as modem only (not in bridge mode) and EERO is in bridge mode. WiFi-network is coming from the one set up with EERO.   
 

Thanks for the help l! 

Glad you got it working