Samsung S9 Desktop Mode failure after e/OS T upgrade


I upgraded from S Official to T Community, ( e-2.1-t-20240605406922 ), and it works very, very well. One thing though…

On S Official I couldn’t cast to my television so I bought a USB - HDMI cable and that just worked - I plugged it in and it mirrored with no settings or changes. Now however casting works on T Community, which is great, but Desktop Mode doesn’t!

I have tried selecting Force Desktop Mode in developer options, obviously it’s not the cable as that was working fine before, so what else can I try before I accept that it just doesn’t work? Has anyone else with an S9 got this working?