Hey, I totally feel your pain with WiFi interference. Had the same issue a while back. The go-to app I’d recommend for your iPhone is Netspot for iOS. It’s pretty solid.
This app scans and shows you the nearby WiFi networks and their channels. It makes it easy to find an open or less crowded channel you can switch your router to. Helps a lot in reducing interference and boosting your signal strength.
Check out more about it here: Netspot for iOS.
Give it a go and see if it solves your problems. Good luck!