Correct, you can't use the simple remote. Either of the two Voice Remotes should work.
https://www.roku.com/products/accessories
You can also find them at stores like Target or Best Buy.
Before you get a remote, the 3810 uses the USB cable with the Long-range Wi-Fi® Receiver . Without it, no remote will work. Assuming it's present and working and the stick is connected to your network, try the Roku app on a phone or tablet.
Roku Mobile App - Free for iOS® or Android™ | Roku
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