I found this issue being related with having Google Keyboard (GBoard) as default keyboard on my Note10+
To fix it:
1. On your device, set Samsung keyboard as default on-screen keyboard
2. On your computer, restart DeX
3. Test that the keyboard works in DeX (it should)
4. On your device, select GBoard again as the default
I'm using an iMac, just tried connecting my Note 10+ to use DEX for the first time. My keyboard didn't work. I'm using an Apple keyboard Bluetooth.
I discovered I had to enable Samsung DEX and AEServer in the Security & Privacy>Accessibility tab. That worked finally. I think the AEServer was recently added with the latest MacOS update.
Hope that helps!!
This worked for me like charm. I would suggest other people who is facing this problem try this just give permission to samsung dex that's it... Thank you so muh sir...