The rumour / article in highly respected UK newspaper was that the Russian secure comms relies on 4G mobile communications. Unfortunately for them these are being destroyed by the artillery attacks, so the Russians are having to rely on older, insecure technologies, including analogue radios in some cases.
"The loss of top ranking officers has come at a time when much of Putin’s invasion force has become bogged down by logistical problems, poor morale and Ukrainian resistance. The failure of its encrypted communications system could be another severe blow.
“In the call, you hear the Ukraine-based FSB officer ask his boss if he can talk via the secure Era system. The boss says Era is not working,” Grozev said on Twitter."
https://www.theguardian.com/world/2022/mar/08/vitaly-gerasimov-second-russian-general-killed-ukraine-defence-ministry-claims