Can an individual be made to use a 3rd Party App such as WhatsApp to communicate?

Here at my unit, everything is basically communicated through GroupMe or WhatsApp. Just for putting out daily information, things needed from the soldiers, accountability, etc. We did the switch from GroupMe to WhatsApp, and all my soldiers were on board with the switch. I have one soldier who may or may not be a very bitter, salty, 12 year E-5 who’s under me (picked up my 5 in November), who initially said they’d rather utilize WhatsApp since the beginning. When information is put out or questions are asked, they deliberately text me separately from our squad group chat rather than responding to the chat we use for the four of us. Today they even had one of my junior soldiers answer for them in response to a question I asked which was very basic, “How long have you been here on station?” for a slide the PSG needed updating. Granted, it could have been found on their ERB which is in their counseling packet.. yes, but it was quicker and more efficient to just shoot the message out. They do this with everything. Sometimes the individual doesn’t acknowledge any messages whatsoever. My CPL and my SPC have no issues communicating, and always keep me updated on things and send “Roger” or “Ack” when I put things out. But the SGT refuses to. Is there a reg or anything stating that if told so by leadership, the individual needs to communicate through that form of communication? I know they’re trying to be difficult, and they’re bitter about the situation they’re in, but it’s because they put themself there. Is it a complete lost cause and I just have to deal with it until they RCP in 10 months? Or can I legitimately put something on paper for them to follow what we literally ALL do? Thank you all in advance!