We don't know if hand communicators work without having a cooperative starship act as an amplifier or router.
I mean, we've never seen Starfleet hand communicator to hand communicator contact when the mothership is out of range. We've never seen Starfleet hand communicator contact from within the ship to any outside asset. But admittedly, we did see Klingon hand communicator contact with outsiders in "Elaan of Troyius"... Then again, nobody was deliberately trying to prevent communications on that occasion.
But yeah, that would be one of the possibilities for thwarting M-5.