In the wiki page of LyX you can see:
How can I have roman pagination (i, ii, iii...) for the first part of my document?
If you use a book class, simply insert
at the beginning of the document. This will switch page numbering to
roman (i, ii, iii...)
will switch back to arabic (1, 2, ...) and start from "1".
With report or article classes, insert at the beginning (in the LaTeX
and then, where you want to switch back (somewhere on the page in the
normal text), in ERT (Ctrl-L):