I believe that having a cell phone is necessary for soldiers. However, I do not believe they should have access to everything. Having a cell phone out in the field could be used to connect with superiors or even equals, but putting themselves on Instagram, FaceBook, and the like is not necessary. Using a cell phone for social media as a soldier could put himself and other soldiers at risk.
Well, correct me if I am mistaken, but if you are going on a mission as an undercover soldier, you are supposed to look exactly like a civilian, and a cell phone is a very important part of today's common man, so I don't see a problem with them carrying one.
If the purpose of "undercover" agents/soldiers is clandestinely infiltrate the army of the Ukraine, then the use of cell phones and social media should be disallowed. The Russian officials should have checked the contents of soldiers bags before deployment in order to avoid problems like this. Social media and secret operations don't mix.
Even the most secure cellphone isn't truly secure. By accessing local and regional cell-towers in order to make or receive a call or text, the solider is risking giving away his position and/or giving the enemy the means to triangulate the location of the remaining forces. In the event of a capture, the phones call and text history to yield critically damaging contact details, of current and/or future operations in the area, as well as security information or access to secured information.