In LO-Calc help for a page break the instruction is go to choose Insert - Manual Break. But I have found that the instruction is actually held at Sheet > Insert Page Break - but this doesn’t seem to do anything at a selected point in a spreadsheet. My version of LO is LO 5.1.5.2 for Linux and otherwise seems to work perfectly well.
Please test how is the option Menu/Tools/Options/LibreOffice calc/Print - Always apply manual breaks.
It does insert a (column or row) break, usually visible as a thin blue line. If not visible check Tools → Options → Calc → View, Visual Aids, “Page breaks”. If in doubt, view your document in Print Preview.