I have same issue using DECO X20-4G Router with WiFi6. Can not add devices to eWeLink account. They are added but no handshake and offline in the app iteslf. OBS it works fine with using my smart phone as a Hotspot. Did a little digging and first reason is.... If some devices experience problems connecting to the Wi-Fi 6 network, the problem may be in missing WPA3 support or the "WPA3/WPA2 mixed mode" feature. Mixed mode is intended to ensure that older clients can still connect, but in practice there are some clients who do not recognize this setting and refuse to connect at all. I went and checked in my router and WPA2 is only activated protokol. Not sure if mixed mode signaling is being signaled anyway?? Waiting for TP-Link to get competent answer. However.... A MAC address is made up of 48 bits. And those 48 bits, they're divided into two different groupings, 24 bits each. The first 24 bits, those are unique to the specific manufacturer or vendor. Those bits are called the OUI, or organizationally unique identifier. The OUI is also commonly referred to as the vendor code. Maybe eWeLink uses MAC filtering, MAC-binding??? Not sure. eWeLink support is not answering...now for 2 days allready. Since I can not connect TCP sniffer to check what is going wrong, I will test with another router of same type different manufacturer, and only WiFi5 available without WPA3 support and see what happens. If anybody has a clue and have resolved the problem, plase share! OBS ALL my devices are working appart from SONOFF modules for HVAC controll.