Angels are active in the book of acts, opening prisons (5:19; 12:7-11), directing preachers (8:26), assuring the Roman centurion Cornelius that his prayers are heard (10:3; 11:13).
After chapter 12, angels virtually disappear. There are references to angels in 23:8-9, and Paul says that an angel assured him that he would stand before Caesar (27:23-24). But we never see an angel doing anything after the angel releases Peter from prison.
Angels are Old Covenant mediators and helpers. For both Peter and Paul, angels come to help and encourage at the moment when these apostles are about to bear witness to Romans. Angels accompany the apostles through the transition from ministry to Jews into ministry to Gentiles, but after that their work is done.