The CMB rest frame is the frame of a comoving observer, that is, one who is at rest with respect to their (cosmologically) immediate surroundings. At different locations, the CMB rest frames are different. There's no global "center of momentum frame" if the universe is homogeneous, only local ones.

(I should also note that in curved spacetimes, reference frames only make sense locally. However, this is more minor consideration, in a certain technical sense. While the impact of the difference in the velocities of different comoving observers scales linearly with their separation, the impact of curvature scales as the square of the separation. So the latter only becomes important at very large separations.)