I bought a pioneer x8500 stereo for my car with an HDMI output. I purchased what looks like an official MHL adapter on Amazon for $28. Each time I plug the cable into the Pioneer I completely lose all mobile data, I cannot even make or receive phone calls and have a circle with a line through it in the notification bar. I can see my screen on the headunit and do everything, even use Wi-fi but no mobile data, which is really important! When I unplug it I get it all back. I tried the MHL on a TV using the HDMI and I can see the screen same as on the pioneer but still no network until I unplug it.
I have a SCH-i545 rooted S4 running 4.3. Kernel 3.4.0. SE for Android status is enforcing. I had the verizon antivirus installed but removed it. I don't know if its something on my phone preventing the connections or the adapter is defective.
I have tried everything I can think of, even wrapping the adapter in tinfoil but nothing helps. I have a Garmin external GPS plugged in below the stereo and the satellite loses its signal when this is plugged in. It may be a coincidence but I have never had that happen before.