Not sure if this will help, but I had the same problem with my iphone 7 and macbook air (and for me too the issue was 100% only with the iphone, all other external peripherals did not have the problem). When I connected the iphone it would just go connected/disconnected continuously. Charging from outlet worked fine too.
With another lightning cable the problem fully went away and with the problematic cable, strangely, flipping the tip of connector would help too. I also recommend cleaning the tip of the connector with a sheet or cloth and maybe the port itself too (you can also try taking a needle or something similar to scrape out any dust that may be compressed there).
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Also I should mention that the “problematic cable“ worked fine for another iphone
Not sure if this will help, but I had the same problem with my iphone 7 and macbook air (and for me too the issue was 100% only with the iphone, all other external peripherals did not have the problem). When I connected the iphone it would just go connected/disconnected continuously. Charging from outlet worked fine too.
With another lightning cable the problem fully went away and with the problematic cable, strangely, flipping the tip of connector would help too. I also recommend cleaning the tip of the connector with a sheet or cloth and maybe the port itself too (you can also try taking a needle or something similar to scrape out any dust that may be compressed there).