I’ve read that iPhones do not record at a constant frame rate, so syncing with audio that is constant won’t align. Here’s an article about processing your video first to get it to a constant frame rate. https://www.switcherstudio.com/blog/fix-lip-sync-issues-with-iphone-video
Someone has already suggested it but I also use the M-Wave Chocolate pedal (here). It's very robust and a great price. I set it up by putting in "keyboard" mode - assigning each button a letter and then having the corresponding letter set up in mainstage shortcuts. Works like a dream. Just decide whether you're going to go wireless with bluetooth or plug it in with usb. If you have it connected with both it will send a double message.
I use this to change my patches. Found it a couple of weeks ago, absolute steal! I use it wired rather than bluetooth just for security. Make sure you don't leave it linked on bluetooth when wired or it will send a double signal and jump 2 patches each click.