how to make input monitoring effects available on ch 3 &4

In the hardware tab of the cubase mixer I have the GUI for accessing rev x and the channel morphing compressor on channel 1&2 of my UR 28m.

The hardware section of inputs 3&4 is totally blank. I use external pre-amps so I want this on my line inputs, how to make this happen.

Which leads to my other question:
On inputs 1&2 does the pad button change it from mic to line level and by pass the built-in mic pres?

There is a limitation to how many channels you can use the DSP FX on. If you disable the DSP on channel 1+2, do the DSP become available on 3+4?
The manual also has a paragraph which states which effects and how many instances can be used. It may be what you are experiencing.
BTW, I have a UR44 so it might be a little different.

Yes I remember that there is a limit, but how do you move it from 1&2 to 3&4?

Remove DSP from 1+2 first then add to 3+4?

I have a UR44 so it may be different for you but I just checked and it does let me enable the channel strip on inputs 1-4 all at the same time so maybe your issue is something different.