Here is a good troubleshooting instruction for you. https://www.tp-link.com/us/faq-2170.html

You can try to change the wireless channel on your main router. Then, restart your router and range extender. Hope it work~

There shouldn't have smart plug with dimmer funtion. Never heard that~

Generally speaking, it should be TP-Link extender as you said and there is no SBL-EXT. Not sure what's wrong with your product, but it seems it's not from TP-Link. It's better to return the unit and...

rajivhifi wrote Model : Hardware Version : Firmware Version : ISP : Deco M5 with ethernet backhaul, 1 main unit and 3 APs. Hardware: US ver 1.0 Software: Latest as of today. Have had this for a month...

For this spec info, you may send email at support.usa@tp-link.com to consult from tech support.