Matthew 24:31 is after Jesus has destroyed the kings of the earth armies that will have gathered at Armageddon to make war on Him and His army.Jesus returns at the 6th seal for a harvest. He sends His angels to gather the elect. Does this look like Armageddon?
Matthew 24
31 And he shall send his angels with a great sound of a trumpet, and they shall gather together his elect from the four winds, from one end of heaven to the other.
Matthew 24:29 is the sixth seal event.Matthew 24
29 Immediately after the tribulation of those days shall the sun be darkened, and the moon shall not give her light, and the stars shall fall from heaven, and the powers of the heavens shall be shaken:
You are (mis) understanding the phrase "tribulation of those days" to mean that the entire great tribulation is over at that point.
There will still be great tribulation following the sixth seal event, such as the rape and pillage of Jerusalem in Zechariah 14:2. Plus all of the armies that Jesus will destroy, resulting in the great feast in Revelation 19:17-18.
Revelation 19:
17 And I saw an angel standing in the sun; and he cried with a loud voice, saying to all the fowls that fly in the midst of heaven, Come and gather yourselves together unto the supper of the great God;
18 That ye may eat the flesh of kings, and the flesh of captains, and the flesh of mighty men, and the flesh of horses, and of them that sit on them, and the flesh of all men, both free and bond, both small and great.