Change stock Status

Hi. On a product we can have 3 different states, regarding the stock status

in stock : a buyer CAN add it to cart
out of stock: a buyer CANNOT add it to cart
on backorder:  a buyer CAN add it to cart

How can I change the stock status to backorder with the plugin? (I can do it manually but not with the plugin)
Is it possible somehow?

I can see the backorder column but it doesn't seem to work, or there is sth wrong with the way I do it.

When instock = true and backorders =yes then I can see the add to cart button //  message= available
When instock = false and backorders =yes then I CAN'T see the add to cart button // message= not in stock
When instock = false and backorders =notify then  I CAN'T see the add to cart button // message= not in stock

If I do it manually, I can set the STOCK STATUS to either in stock/out of stock/ on backorder

If I choose on backorder then I can see the add to cart button with a different message than all the above.