What should a traveler do if he misses the Maghrib prayer while traveling, and he has [already] settled down after the Isha prayer?

If you arrive at your place of residence (town or home), you must pray Maghrib and Isha in full. However, if you are still within the travel distance, you pray Maghrib and Isha shortened, unless you intend to stay for four days or more while still in travel, in which case you must complete the prayer.