How to recover footer and slide number areas in slide master

Somehow, my slide master lost the areas I had defined for footer and for slide number.
So the values do not appear on the slides. Is there any way to recreate/recover from this, keeping the current non-Master slides?

Please look here if it helps you:

My Master Slide in Impress

Thanks for the pointer, but all actions there seem to require clicking on the footer box which is now missing.

You’re right. In the new version Version: (x64)
Build ID: 7cbcfc562f6eb6708b5ff7d7397325de9e764452
CPU threads: 8; OS: Windows 10.0 Build 19041; UI render: Skia/Raster; VCL: win
Locale: de-DE (en_DE); UI: de-DE
Calc: CL
it has probably been changed.

In the master view click on Date/Time in the footer and select Edit Template.
Close master view.
Choose Insert>Header/Footer.
Enter your data.
Click on “Apply to all” or on “Assign”.

I hope this helps.