To reproduce:
1. Open attached project "case_1082184-WebSocket_Tester_Simple_2 2.zip"
2. Open "Main.unity" scene
3. Enter Play mode
4. Make sure that WSS is enabled and then press SEND
5. Observe on the right of the button as the connection was established successfully
6. Build for iOS
7. Open the Xcode project and deploy to iOS device
8. Make sure that WSS is enabled and then press SEND
9. Observe as the connection could not be established

Expected result: connection is established
Actual result: could not establish a connection

Reproduced on:
2018.2.0a5 2018.2.8f1 2018.3.0b2 2019.1.0a1

Not reproduced on:
2018.1.9f2 2018.2.0a4

Device reproduced on:
iPhone 7 Plus iOS 11.1.1

Notes:
Couldn't reproduce on:
Samsung Galaxy S9 8.0.0 Snapdragon 845 SDM845 Adreno (TM) 630 OpenGL ES 3.2
with Unity 2018.3.0b2
According to the user, this issue also is reproduced on an Android device.

WS toggle works in Editor and on an iOS device.