The WiFi band that your Echo speakers connect to, shouldn't matter. I have all of my speakers working in multiroom groups despite some being on 5GHz while others 2. 4GHz. I use the same SSID & Key for both bands to allow my Netgear Nighthawk router's Band Steering function to work. However if running many speakers, it would probably be better to use 5GHz if practical, since its faster connection speeds offer more bandwidth to be shared among them all.