For about a month or so Screen Mirroring has been broken for the one App that I primarily use it for: Netflix streaming via the Netflix App. I have spent hours/days/weeks troubleshooting and researching the problem online to find no answers posted. I have even, regrettably, factory reset my phone in an attempt to find a fix. T-Mobile Support and Samsung Support were baffled. After much personal troubleshooting on my own I finally found that the problem is not with my phone, not with T-Mobile, but with the Netflix App update recently pushed out. The update had no warning to End Users that the update would break functionality on existing devices and so the End Users accepted the update, unknowing that it would devalue the App as a video content provider from that point on.
In an attempt to secure licensed content, Netflix App developers added HDCP code to the App which broke compatibility to any device that does not have code that supports HDCP, such as the Samsung AllShare Cast dongle and the ActionTec wireless dongle.
Screen Mirroring with the Samsung AllShare Cast dongle or other such device will continue to function perfectly with other Apps, such as the YouTube App and Flixter App, but will no longer work with the Netflix App.
Netflix Support states that the Netflix App developers have no plan to fix the break and have no plan to provide a patch to support what they referred to as "old tech." According to Netflix App Support, devices which support HDCP, such as the $1500+ Samsung TV, the Visio TV and Google ChromeCast dongle ("Quantum Leap" technology), will continue to work with the Neflix App, at this time. There is no known End User-side workaround, that I have found and I do not believe rooting the phone will do any good at all, since the problem is with the Netflix App and not the phone itself. (Nor do I support rooting the phone, but I wanted to warn anyone who may be tempted to do so.)
I have a Samsung Galaxy S5 and my husband has a LG.