Anyone familiar with color spaces and calibration help me out?

Heyya, I usually publish my photos online, and *most * of my clients view my images on their imacs/iphones.

I recently purchased a Dell UP2716D and am having trouble setting it up. I tried setting it up as srgb, and used my calibration hardware to fix it. When I edited my photos and sent them to my iphone to be viewed, they did not look the same.

What color space do I setup the monitor as srgb/adobergb/p3/rec-709 to best match an imac display?

Thanks for your help