Sounds like you have two DHCP assigning devices on your network.  Only one router should be assigning addresses, the other should be pass-through only.  I typically see people taking their ISP modem/router combination and disabling the router (set it to pass-through only), then feeding their internet into their own router - in this case your eero.  When I was on cable, I refused their hardware, bought a simple cable modem and fed the output into my eero.  Clean, simple, and cheaper in the long run!  Either way ONE device handing out addresses ONLY!  Good luck!