Unless your brother’s WIFI has the same SSID and password as you do, you will need to reset the Play 3 in order to set it up in his home. Then you will need to repeat those steps once you get back to your home.

One of the things the new app app still cannot do is change the WiFi credentials on older speakers without a reset. See https://support.sonos.com/en-us/article/connect-sonos-to-a-new-router-or-wi-fi-network So to use the Play 3 at your brother’s you would have had to reset the Play 3, or change his WiFi to your credentials.

You did however probably not reset the speaker, since it did not work at your brother’s. It is added to your network now, but you can’t reach it from the Sonos app, only from Spotify?

I would restart the router and all speakers. If that does not help, reset the Play 3 and ad it to your system  (do not start a new system). See https://support.sonos.com/en-us/article/reset-your-sonos-product for how to reset.