This is simple, folks. St. Paul spoke of the resurrection, or the Rapture, happening at the "last trump", as quoted in 1 Corinthians 15:51-52:
51 Listen, I tell you a mystery: We will not all sleep, but we will all be changed— 52 in a flash, in the twinkling of an eye, at the last trumpet. For the trumpet will sound, the dead will be raised imperishable, and we will be changed.
There is also a mention of this trumpet in 1 Thessalonians 4:15-18:
15 According to the Lord’s word, we tell you that we who are still alive, who are left until the coming of the Lord, will certainly not precede those who have fallen asleep. 16 For the Lord himself will come down from heaven, with a loud command, with the voice of the archangel and with the trumpet call of God, and the dead in Christ will rise first. 17 After that, we who are still alive and are left will be caught up together with them in the clouds to meet the Lord in the air. And so we will be with the Lord forever. 18 Therefore encourage one another with these words.
1 Corinthians was written approximately between 53-57AD, while Paul was in Corinth. 1 Thessalonians was written earlier, around 52AD.
The Apostle John was given his Revelation by God on the Isle of Patmos in 95-96AD.
Since John's Revelation was written so much later than Paul's letter to the Corinthians and Thessalonians, they would have had NO idea what Paul was talking about if he referred to the last trump of Revelation.
However, being Jewish and/or interacting with Jews, they would have known the "last trump" referred to a specific ceremony, namely, the Feast of Trumpets, when 100 blasts of the shofar is made, and the very last shofar blast is loud and held for as long as possible. It is called the Tekiah Gedolah.
Everyone Paul preached to in that day would certainly know what the "last trump" meant.
They would not have known about the final trumpet judgment as recorded by the Apostle John decades later, and therefore cannot be one in the same.