can't use draw material to render target in ios

hello, i use draw material to render target node in my blueprint, then show this render target to a plane in my level. however, when i cook it to my iphone8 plus, the render target is black no matter what i render to it. Does anyone has the same problem, or i miss something? many thanks. btw 4.21 is fine.

use metal desktop renderer when packaging

i tried this but doesnt work