Under The Waves

On day 2, you'll get asked to go back home to try out crafting. Once you've done that, you'll get a radio message telling you to check out the comms - first an antenna on the roof of your base, then to the antennas themselves.

An icon will be added to your map, a little north of your base. Head there, and you'll be tasked with investigating the communications problems. Whilst you're there, press the button on the base of one of the working antennas and this should pop.