Spoiler: show
The GoT universe actually provides a mechanism by which messaging could take place in near real time. A warg stationed at, say Eastwatch, could inhabit a bird to keep a watch over the group and then report back on what they see. Or if there were a warg in the group, they could warg into a bird back at the Eastwatch or Dragonstone and the bird could peck out a simple ouija board message. With magic on the rise you would think there that every side would be scouring their forces for naturally gifted warging individuals which would be a huge strategic advantage. Or at the very least, Jon Snow's side would, his having taken in the Free Folk and having seen warging in action. Beside that, it was stupid that he didn't take a larger force. This was a Hail Mary pass of a mission. The whole point of which was that capturing a wight was necessary to convince Cersei. It had to succeed, we were led to think. On top of which, Jon's death during the mission could jeopardize everything -- the Northern coalition, plus any chance that Dani would be motivated to help his people. So why not bring enough people to ensure its success? Then, at least, it would make some sense their encampment could hold off the army of the dead for a few days. Anyway, it seems clear that the Night King held his forces back on purpose. He must have the greensight or some comparable gift of prophecy and was able to foretell that Dani would bring her dragons. So he needed Jon's people to stay alive long enough for Dani to attempt a rescue, and then he was ready to strike Viserion down.