Bluetooth USB dongles are not supported and don't work.
Which RF USB keyboard did you try? Many find that Logitech K400 line to work.
I have a Samsung UE40JU6000K smart tv. Unfortunately it does not have integrated bluetooth, but it does have wi-fi direct.
The TV works fine with wired USB keyboard and mouse. But that's not very useful if I want to use the keyboard for iPlayer searches, internet browsing etc from the comfort of my armchair.
I've tried two wireless keyboards, (one bluetooth, one RF - both came with USB receivers which I plugged into the TV) - neither worked. Although the tv doesn't have integrated bluetooth, I thought it might still work with the USB adaptor/receiver plugged in, but not so.
I'd like to know if there is such a device that can be plugged into a USB port as a proxy to make the smart TV (or anything else for that matter) think the wireless keyboard is a wired keyboard plugged in directly.
I know it's a long shot. Does anything like this exist?

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