Dual Display with MacMini 2019 and Big Sur

Hello
Using. MacMini 2019 model
I upgraded to Big Sur and one monitor is blank.
Had no problems with Catalina at all.
So looks like a OS issue.
Can anyone suggest a solution? or Have I got to revert to Catalina?
KM
Dual Display with MacMini 2019 and Big Sur
 
 
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