It is ideal to have the router in modem only mode but if the eero is having drops every 15 minutes, something isn’t setting up right. Tough to know if its the eero doing it or the arris since, in theory, the arris is only acting as a modem.
But if you can leave the arris in router mode and put the eero into its DMZ, it should auto-forward all the data to the eero.
The ip address will be dependent on what the eero chooses during setup.
I would try that route. You can leave the wifi on the arris or disable it. I usually leave the wifi on and just make sure its on a different channel. Then I can offload devices or take a little load off the mesh system.