I have a similar problem. I want to start as a one column and then change to a two column layout for a couple of pages. The frustrating thing is that whenever I apply any page style to a page it applies that style to the entire document. This happens across sections as well as across pages.
When you go to Format → Columns you are going to change the column setting for the default page style. So when you change that all the pages change.
Also surprising is that I do not seem to be able to apply a custom page style to one page. At least for me, when I apply a page style to any page it applies it to every page in the document, even if there is a manual page break before that page or if pages are in a separate section.
The thing that worked for me was to leave the Default page style one column and make a custom Two_Column page style.
Where I want to shift to two columns I add a manual page break (Insert → Manual Break) and in the style drop down choose my custom Two_Column style. When I want to go back I do the same thing and insert another manual page break with the Default style.
LibreOffice Version: 18.104.22.168