1 After these things I looked and saw a door opened in heaven, and the first voice that I heard, like a trumpet speaking with me, was one saying, “Come up here, and I will show you the things which must happen after this.”
after these things – that is: after the LORD’s Day, this statement signifies that John has been transported to the New Heavens and the New Earth. John’s entire meeting with the Spirit of Christ from Rev 1:7 to Rev 4:1 was in the LORD’s Day. Now the Spirit of Christ calls him up into the New Heavens. This is clear from verse 4, and Rev 5:13.
The first voice which I heard like a trumpet – from Rev 1:10, the voice of the Spirit of Christ, the Angel, Rev 1:1, who is guiding John through this Revelation.