One other possibility that hasn't been mentioned is whether Apple CarPlay is available in the car.
CarPlay in some vehicles will take priority in connecting the iPhone to its wifi network. I have CarPlay in my Mini, and I have found that the easiest way for me to connect to my A139 pro is to disconnect the iPhone from CarPlay first. I do that by turning off CarPlay entirely in the iPhone's Settings–>General–>CarPlay–>Mini menus. Once I turn off CarPlay, I find that if the A139 Pro's wifi has already been "Enabled" (by turning wifi on with the wifi button on the camera), the iPhone connects almost immediately to the dashcam on its own.
Apple is known for creating (or trying to create) seamless integration for its own ecosystem, and I'm not surprised that the iPhone connects first to CarPlay... there's probably nothing Viofo (or we) can do about this if people want to connect to CarPlay.
After I'm done with transferring dashcam files or whatever else I need to do with the camera using the Viofo app, I re-start recording on the camera, turn off wifi on the camera ("Wifi disabled)" by holding the wifi button on the camera, and then turn CarPlay back on in the iPhone. It's a bit of a pain, but the iPhone is only going to connect to one wifi network at a time and Apple just seems to be trying to make connection between the iPhone and CarPlay in the car "seamless" (automatic).