Hey,
The option is right here, after clicking it mention the min and max thickness in your model. Since your model seems to have a uniform single thickness you can mention a small range (say the thickness of wall is 2 mm - min=2 max=2.5 ). After that box select the entire model. The software will search through the entire model and automatically select the offset faces. Even if some faces are shared between cells as long as that wall is generated once it should not be a problem for you.
Let me know how it goes.
Regards,
Mohan Urs
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