Custom Screen resolutions

I have a Mac mini connected to a Dell monitor which has a max resolution of 1920x1080 however when I mirror a MacBook display I can increase the resolution to 2560x1440. Can anyone advise how to add this resolution when not mirroring the MacBook display?

Thanks

 I can increase the resolution to 2560x1440.       Resolution of MacBook or Dell display ?

What kind of connection is it, between the Mac Mini and the Dell monitor? What cable?

Its an HDMI connection between the mini and the monitor and wireless mirroring when connecting to the MacBook. So I know the monitor can handle the resolution as it does it when replicating the resolution from the MacBook but when disconnected its not an option.

I've also tried easyRes and SwitchResX but neither allow me to add the custom resolution to the mini.

Thanks

Custom Screen resolutions
 
 
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