I am having the same issues even with the newer Kernel, 4.4.0-79-generic.
Like others have mentioned above, I can mess with the settings and sometimes "it gets it" and other times it just causes the whole machine to lock up and I have to force reboot it. Even when the monitors do show up in xrandr - they are blank with the no signal screen. Messing with the settings becomes a 50/50 but even if it does "manage to find it", the moment you have to walk away, or the screen goes to sleep, its not coming back. Here is a screen shot of the error I get sometimes, other times, there is no error and the machine just locks.
I am also on a Dell E7250
Intel Core i5-5300
Intel HD Graphics 5500 Broadwell GT2
I am having the same issues even with the newer Kernel, 4.4.0-79-generic.
Like others have mentioned above, I can mess with the settings and sometimes "it gets it" and other times it just causes the whole machine to lock up and I have to force reboot it. Even when the monitors do show up in xrandr - they are blank with the no signal screen. Messing with the settings becomes a 50/50 but even if it does "manage to find it", the moment you have to walk away, or the screen goes to sleep, its not coming back. Here is a screen shot of the error I get sometimes, other times, there is no error and the machine just locks.
I am also on a Dell E7250
Intel Core i5-5300
Intel HD Graphics 5500 Broadwell GT2