It’s strange I had these same connection issues when I first got my iPhone 7 Plus and spent ages trying to figure it out, even to the point of nearly returning the phone. Anyway I found that if I connected everything in a specific order it would connect, strange but it works for me. The order is listed below, be real interesting to see if this works for someone else.
Turn on controller (without phone connected)
Turn on Mavic (wait until controller connects)
Plug in phone (no Go 4 running at this point)
Run Go 4 app