I posted the same problem on a new thread, and found the solution this morning on reddit.
I plugged the phone into my portable monitor this morning, turned on my BT mouse and keyboard, told my friends to check out this cool new way to work, and left them unimpressed.
I can only use the mouse on the phone screen.
Dex comes on and the BT keyboard works with it. I was able to tab my way to settings and found the mouse options were missing.
Cleared cache and data from all of the dex apps.
Unpaired and repaired the mouse.
Restarted the phone.
Tried phone as pointer. Pointer is still trapped on screen.
This seems to be exactly the same problem as this guy from a couple of months ago: https://us.community.samsung.com/t5/Note10/Dex-mode-mouse-only-appears-on-phone-not-the-external-dis...
His problem was not resolved.
From user Redflea on r/SamsungDex:
"I was able to get this working again. I have been using the Dex on PC app at work and it seems that sometimes after removing it from the PC app it will not work properly on a Dex dock. I had to plug it back into my work pc, stop dex, and then it worked as expected on my standalone monitor with dex dock."
I connected my phone to my PC, let it auto launch Dex, closed the Dex application on Windows, then disconnected my phone.
The mouse works in Dex mode on my portable monitor again!