The Russian Military of Defense introduced an encrypted communications platform named Era back in 2021. It's supposed to work everywhere, but apparently it has a dependency on 3G. Russians have destroyed many 3G cell towers in Kharkiv and deployed stingrays, which has inadvertently stopped Era from working for many of their forces.
Per the discussion in the intercepted call, the FSB officer assigned to the 41st Army informs his boss that they've lost all secure communication.