It’s called Blackspot, the fungus causes circular black spots on the surface of leaves. It tends to thrive in warm, wet climates with high humidity.
You can use Funginex, Orthenex, or Daconil.
You will also want to pick up any fallen leave and dispose of them in the trash.
If you just paid good money and that was delivered I agree totally that you need to get your money back. It has an awful case of blackspot and should never have been sold in that state. It is neither here nor there whether you can mend it. If you study the roses catalogues it might pay to see which ones are more resistant to blackspot - they would be easier to care for.
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