Like both are taken maybe a 3-4 seconds apart but both are around 5 minutes long. Is there a way to make it sync with the video taken with an iPhone ? Not sure if this question makes sense.
You can do it in post with a video editing program like Premiere, Vegas, iMovie, etc. There's also software that's specifically designed for this task like: https://www.redgiant.com/products/pluraleyes/
Making a loud noise or clap at the beginning of the clip can also be helpful with the syncing in post.
Slate the take and sync in post. You can do a audio slate like the post above suggests but i would do a timecode slate on both cameras then you just line up the numbers. Easy
Yeah like the others said above, old school is still the best school. Just make a loud noise on both and sync the waveform spike in post or clap on the video and sync the sound on the external to when your hands meet. Easy and cheap, the best combo there is!