Time passed. It wasn’t instant.

I assumed that the Starfleet ships were prepositioned near the barrier/wall ready to cross. We were shown in the previous episode that there were old remnants of Borg Transwarp conduits in the region.

Reno could have messaged Vance even if full holographic communications weren’t available.

The episode makes clear that hours had passed while the Reno and the cadets on Athena were repairing the ship and working out the solution, then another 30 minutes to get to the star system at a limping warp speed and more to find Brakka.