I do the same thing as Thegent - Guitar to Reaper, then video on my iphone. I pull both recordings into Davinci Resolve and let the software sync the audio tracks. Then I just disable the included audio from the video track.
From iphone there is an intermediate step because video is variable frame rate, it has to be converted to constant frame rate. Some phones allow you do this when recording, but iphones don’t. If you don’t convert to CFR, when you render the video, the audio from Reaper will fall out of sync with the video.
Good luck! It is way more complicated than it should be, but once you get it, it sticks…kinda like playing guitar!