Easy. Just change the name (SSID) for each band of your router. For example, name the 2.4 GHz band Home24 and the 5.0GHz band Home50. Then set up your Echo devices to the band that you want.
Your router is correct in stating that all your devices are on one network. You have one network with 2 bands (a 2.4 band and a 5.0 band). Changing the band does not change the network in this case.
If your router sets both bands to the same name (some routers do this, namely Google Wifi), there isn't a way to instruct your Echo devices to choose a different band. It's just done automatically. And most devices will fall back to the 2.4GHz band since it's usually stronger than 5GHz.