Have a PC set to DHCP plugged into the Eero.
Open a command window and type in "ipconfig".
Note the "gateway IP" address.
Open a browser on the PC to that IP address.
The Eero's admin login username/password (and possibly its IP address!) should be printed on a label on the bottom or the rear of the Eero.
Look for the router's LAN settings, that's where the router's IP address (LAN / gateway) will be located and where you can change it.
There's posibly a decent manual for that router on their support site, either from Eero or your ISP => Support in the UK: +44 800 032 2183.