The cheapest way would be to go to your WiFi router and look for "Guest WiFi" settings, hope it's not too cheap that this functionality isn't included, activate

The cheapest way would be to go to your WiFi router and look for "Guest WiFi" settings, hope it's not too cheap that this functionality isn't included, activate said network, and put the devices on the guest WiFi. More complicated settings involve the keyword "VLAN", afaik most home routers don't have this.