Matthew 24:31 does not teach a post-trib rapture.
I don't see a problem between Matthew 24:31 and a pre-trib rapture.
The gathering of the elect in this verse is not the rapture. The catching up/harpazo will happen as part of the same event as the resurrection of the just and the angels will not be gathering those in Christ to meet Jesus in the air (1 Thessalonians 4:15-18).
1 Corinthians 15: 51-52 confirms that the bodies of those living [in Christ] will be changed. They will not be carried up by angels to meet Christ in the air.
1 Thessalonians 4:15-18/1 Corinthians 15:51-52 and Matthew 24:31, are two separate events, separated by at least seven years. The elect in Matthew 24:31 are not the ones "in Christ," who are changed from mortality to immortality in the twinkling of an eye but are mortals who come to Christ during the Great Tribulation, survived it, and are being gathered by the angels and taken to a specific geographic location, possibly to Jerusalem where the marriage supper of the Lamb will take place.
Read more about it: https://www.amazon.com/Redemption-Bible-Prophecy-Simplified-Study/dp/170261946X/