Ubuntu 20.04 - Simplescan ignores feeder on OKI ES8453 MFP

The volontary association, I am trying to help, had the local autorities to install Ubuntu 20.04 on four desktops on ethernet.
The MFP printers were installed automatically and print okay.
But scanning from PC only works from flatbed, even if you order it to use the Document Feeder.
The social authorities should service the Ubuntues, but we have not much hope, that they will use the time it takes to solve the problem, and no one else are allowed to do it.

I cincerely hope to be able to solve the problem myself on my own Ubuntu laptops to be able to show the local autorities how to do it.
But I am stuck.
Our most important MFP is an OKI ES8453 MFP,
and the other one is an HP Color LaserJet Pro MFP M477fdw.
I have just had a try with Ubuntu 16.04; but that only found the OKI automatically.
The Ubuntues have simple-scan installed, but not xSane.