I tried to pair a sonoff with an Iphone x with IOS 13.3.1 it doesn't pair, then I tried to download the new IOS 13.4, I tried the pairing again, nothing to do. So I tryed again with an Iphone SE with IOS 13.3.1...it's works!!!
Maybe is a mix with iOS 13 and HW...
Since iPhone SE uses an old HW, the problem never occurs even with iOS 13, but occurs with later models, with newer HW and iOS. F
Quick Fix (iOS 13 Pair issues with Sonoff 4CH Pro):
Step 1. Ensure that your account with eWelink is properly setup and running
Step 2. Install eWelink on an Android phone (borrow from a friend if you dont have)
Step 3. Download eWelink on the Android phone and sign-in with your login details from step 1
Step 3. At this point, follow the screen setup on the android phone to complete the setup.
Step 4. On your iPhone, login using same details as in Step 1
step 5. Refresh Your screen on the iPhone by pulling down as usual and there seats your added device.
1. Deleting the app on the Android phone after the setup will not affect the functionality of your device.
2. You will have total control on the iPhone afterwards.
This approach worked for me after 5 devices has been ordered, shipped via DHL Express (China to Nigeria) and 7 solid hours of trial/error.
I hope it solves your challenge with the device.