When I use my UR824 this is kind of not so important because I just then MIX 2 L/R or whatever to my headphones preamp and it`s kinda ok (although we lost 2 extra mix outputs)
But when I’m working with the UR242 I just have a “MIX 1 L /R” and the headphones just mirror this output! It’s impossible to get a different mix in the headphones. The question is WHY?
Possibly because the Phones are internally dedicated to Mix 1.
You only said UR Series in the title - so - the UR28M has two Phones outs, one is dedicated to mix 1 the second is selectable to any of the out pairs.
In the ur824 (2 headphones) is the same.
Don’t you think that would be better if the mix1 - 2 - 3 - 4 are for different sets of speakers and have a non dependent headphone mix?
Does the 824 have Independent Mode available as does the 28M?
Have you tried setting up Phones channels in the Control Room?
Otherwise please explain your problem and setup with more detail and perhaps a concrete example.
With the 824 you can select a source for the headphones within the available 4 Mixes, those 4 mixers are actually the 4 pair physical outputs of the 824.
In the 242 is the same, you can select “a mix” for your headphone output. Since the 242 have only one physical output, the headphone output is just a copy of your main output.
What I wanted is to have a different and independent for the back physical outputs mix in the headphones.
So I ask again, can you setup the outputs to operate in independent mode?
Ah, sorry my head is a mess today [FLUSHED FACE]
So no, there’s no independent mode
Which brings us back to setting up a phones channel in the control room which can be any outs you choose, and can be switched between main and cue or custom mixes.
Have you considered that?
No, there´s no “Headphone out” in control room
You have to create it first!
Here from the Operation Manual:
You can create 1 phones channel for a stereo configuration. It allows you to listen to the main mix or cue mixes or to external inputs on a pair of headphones. You can also use it for previewing.
I’m ready to eat my hat!
Where do I have to create a Headphone output?
I use Cubase for tracking for ages and never did that, can you point me where do I have to created? (look at my gif file please) 
You probably have the Operation Manual in a language other than English, I can only refer you to Pg,352 in the English Manual - otherwise, it’s the Control Room chapter - section: Adding Channels to the Control Room, then see the section Control Room Channels.
PROCEDURE
1.Select Devices > VST Connections.
2.Click Studio.
3.Click Add Channel.
A pop-up menu lists all available channel types and shows how many instances of each type are available.
4.Select a channel type.
For most channel types, a dialog opens, that allows you to choose the channel configuration.
5.Click the Audio Device column to set an audio device for the channel type.
6.Click the Device Port column to assign a port for the channel.
IMPORTANT
You cannot assign the same device port to a bus or channel and a Control Room channel at the same time.
RESULT
The Control Room functions are available for use. If you disable the Control Room, the configuration is saved and will be restored when you reenable the Control Room.
Hallo BriHar
That’s what I was doing in my gif, and there’s no “headphone” output in the list. 
Sorry, my bad. I caught UR824 at the beginning and latched onto that. The UR242 only has the single mix outputs, and the FX Bus has no real world outputs, so I believe what you want to do is unfortunately not possible with this hardware. Sorry if I got your hopes up. 
I have an 824 in my studio back in Perú, but I will be there in June…
[emoji22]