@logar it’s nothing to do with MAC address filtering that’s for restricting acces to you wifi by MAC address. Setting echo setteing in routers is usually under the LAN settings.
The reason he is suggesting reservations is because the router assigns IP’s dynamically and your sonos is given an IP and then you discover the sonos on that IP in smartthings so when this IP changes Smarthings will still try to speak to the device on its old IP until you do a rediscovery on its new IP.
It’s best to set static DHCP reservations for your whole internet of things otherwise you will hit issues like this. I’m not sure any of us can really assist with assisting on a specific model of router unless one of the members here has the same model. It will be worth reaching out to your ISP’s support as they should be able to assist quickly failing that see if you can source the manual for your specific router and post it here and then the community will be able to assist.