there are news articles (I admit, of questionable origin) the claim he visited Russia yearly for the past decade. I do wonder if it the situation is a little more complex then he was a fugitive of the Russian Government
Even Politico and the WSJ can confirm...
https://www.politico.eu/article/telegram-ceo-arrest-pavel-du...
https://www.wsj.com/world/russia/russia-military-telegram-fo...
It is forbidden for military to military communication, but openly used a lot for information from authorities to the public.
A reason why russia use it is because they don't have anything better.
Ukraine on the other hand had fairly advanced systems well before recent Western military aid started arriving.