This achievement is story related and cannot be missed.
For this achievement you'll need to venture off from Cassardis. After you finish the battle on the beach you'll awaken and start your journey in the town of Cassardis. Here you'll be able to explore the vivid town, take on several quests, prepare for the journey ahead. From there you'll need to head to the main gate to leave the town. Once you've enter the open world you'll pop the 5G achievement!