There's two options, one is to turn it into a string as discussed.

The other is:

Format a cell with a number format such as \0000. This format can be assigned in the Format code field under the Format - Cells - Numbers tab, and defines the cell display as "always put a zero first and then the integer, having at least three places, and filled with zeros at the left if less than three digits".

https://help.libreoffice.org/latest/en-US/text/scalc/guide/integer_leading_zero.html