A friend of mine had Diastasis Recti and she used a belly band and it did help with her Diastasis Recti.
She eventually had surgery though because although the belly band did help somewhat with the Diastasis Recti it didn't fully get rid of it.
The surgery was $5,000.00 but she was able to get a personal loan to be able to pay for the Diastasis Recti surgery and she said it was well worth the money.
But if you cannot afford the Diastasis Recti surgery then yes a belly band will help it somewhat since it will help to close the Diastasis Recti gap.