Hi @Filippo S
Welcome to the Sonos Community!
If you can only connect to Sonos when your app-running device is on your old WiFi network from your original router, then you probably just need to tell Eero not to operate as a router itself - otherwise, you have two networks.
As long as you still have the original router on your network, please follow the steps here to put your Eero mesh into Bridge/AP mode:
https://support.eero.com/hc/en-us/articles/208276903-How-do-I-bridge-my-eeros-
Please reboot your router after doing this to make sure all your network devices have IP addresses from the router and not from your Eero mesh.
Additionally, if you permanently connect one Sonos device (not a Sub, or a Move which can’t) to your network with an ethernet cable, all of your Sonos system will bypass your Eero WiFi network and it’s settings. If you don’t want a speaker in that location, you can try wiring to a mesh node instead of the router, or purchase a Sonos Boost to connect to ethernet instead.