Page numbers not showing in place

[Melvin Bolton]

I have in the past completed a lengthy book, using Lyx,  without needing 
to trouble anybody for help,  but I'm now baffled by what should be a 
simple, basic task and I have no idea what to try next.

My problem is that the pagestyle settings that should place page numbers 
in the footer, are showing them in the output only on the first page of 
each chapter, which the User Guide says is treated by Lyx as a special 
environment.
I began with KOMA-Script ( scrbook) with pagestyle set to 'plain' as I'd 
done previously. This places page numbers in the outer end of the footer 
as I prefer them. When I found them only appearing on the first page of 
each chapter I searched online and tried pasting a few commands in the 
preamble but nothing helped and mostly I got compilation errors. I've 
cleared the preamble.

I tried Lyx Standard Class (Book) with pagestyle set to 'plain', which 
centred the page numbers as it should, but still only on first pages of 
chapters. Neither was there any improvement after I tried pagestyle set 
to 'fancy' and tried to customize that. It's as though chapter footers 
are not accessible, except for first page.

In my document, immediately before the first chapter, I have the 
following ERT:  \mainmatter\pagestyle{plain} \setcounter{page}{1}

The frontmatter is fine because I have don't want page numbers there and 
have pagestyle set to 'empty.' That, at least, has worked; there were 
Roman numerals there to begin with.
Finally, I did a reinstallation of Lyx (ver. 2.3.6.1) in case of 
corruption, (apparently not) and I'm now out of ideas.

I shall be truly grateful for any help, but I will need precise 
instruction if I am to paste commands correctly into the preamble. I 
really am an old novice.

Thank you

Melvin