You need to be level 29 max to enter in Caergate, if you go 30, you can't complete the quest any more.
If you live in a big country with "normal clock", the escort starts at XX:40. It means 1:40, 2:40, 3:40 and so on, every hour past 40 minutes. You need to be near the escort when it starts, near the base in the north corridor. When it check the personnel, you see the number of participants in top screen. If you don't see it or you fail to escort till the end of the road, or some enemy destroy the escort, you miss the escort.