How to resume downloading iOS update without starting all over again?

Hi, I have downloaded (via my Mac) 3/4 of the huge iOS 14.3 update for my iPhone, which used up more than 3GB of my monthly data, before my Mac idiotically went to sleep and the internet connection was interrupted as a result. The only way for me to complete the download seems to be to start from scratch, and I don't have another 4.3GB left for this month!! Is there a way for me to find the partially downloaded update and resume from there, without downloading the whole update from the start? I have already signed in to 'check for available downloads' but there was nothing there. Grateful for any advice, thanks!

I remember that, when downloading a MacOS update, the download would simply resume from where it was last interrupted and not download the whole update from the start. But this doesn't seem to occur with the iOS update.

(By the way, I still can't believe the Mac could possibly go to sleep when it is actively downloading an update from the internet. What an idiotic design! I suppose I could turn off the sleep function before I download anything big next time, but it is unbelievable that such a basic thing is not already built into the sleep function by default. Especially given that the Mac has a heap of other creepy features that make it 'smart' enough to do things that I don't need and have not asked it to do.)