The device must have connected at least once to appear in the "Online Devices" list. If it's a new device that has never connected, you cannot filter it until it attempts a connection. As a workaround, enable "Wi-Fi Access Control" (Whitelist) to block everything new by default.
(Deny List) feature rather than a traditional whitelist-based MAC filter found on older routers. This system allows you to prevent specific devices from connecting to your Wi-Fi by identifying their unique MAC addresses. TP-Link Community How to Filter Devices via the Deco App mac address filter on tplink deco m4
Blocks all devices by default, permitting entry only to pre-approved MAC addresses. The device must have connected at least once
To go beyond basic blocking, consider these expert strategies: consider these expert strategies: