Does the light on the remote start to flash when you press and hold the pairing button for 5 seconds? If not, then we need to start with that problem. Make sure the remote is fully charged as well.
Not being connected to your Wi-Fi wouldn't have anything to do with it. My preferred method for pairing a remote is to disconnect the power from the Roku device, press and hold the pairing button on the remote until the light starts flashing, set the remote down next to the Roku device and don't touch it, connect the power to the Roku device. Once it starts up it will pair. I haven't had it ever fail.
I almost forgot, the 3821 requires the Long range Wi-Fi Receiver. If you don't have that you'll never get the remote to pair.
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