With Streaming Stick model 3810, released in 2017, Roku enhanced the WiFi receiver and moved it from the interior of the stick out into a bulge in the USB power cord. For this and later Streaming Stick models, a specialized power cord with a "Long Range Wi-Fi Receiver" must be used. Without it, the stick cannot connect to the internet, nor can it communicate with its remote.
There are two different power cords with Long Range W-Fi Receiver available, depending on your Streaming Stick model. The two are NOT interchangeable.
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Ultra 4800 | Streaming Stick 4K+ 3821 | TCL Roku TV 43S245/C107X