Google Sheets - Conditional Formatting based on value of cell in another sheet - Custom Formula

how can I achieve the following:

Sheet1 has a range of cells A1:M13 which have conditional format rules: if text is exactly "1" (to "15" so 15 rules) → each number gives the cell a different background filling.

No I want do Conditional Formatting of Sheet2 range A1:M13 based on the cell values in Sheet1 A1:M13 because I don't want text in the cell of Sheet2 so I can put in random text which doesn't effect cell's background etc.

At the moment I have the following custom formula in conditional formatting:

=if((INDIRECT("Sheet1!A1:M13=1")))

What do I do wrong? Do I have to set format of Sheet1 to value=1 instead of text is exactly "1"?

Thanks for any help, guys!