In this paper the Model Order Reduction technique to solve the problem of free oscillations of a heterogeneous rectangular elastic membrane is applied. Instead of solving 2D problem for the membrane in the exact formulation, we substitute it by a special network of 1D elastic strings. We present the characteristic equations for the spectrum of free oscillations of this network and develop the numerilal algorithm to solve the problem. We investigate the behavior of eigenvalues of a rectangular network and show that the eigenvalues and eigenvectors of rectangular networks of elastic strings and the rectangular membrane are close. The problem solution for the network of elastic strings has significantly less computational cost compared with the solution of free oscillations of a heterogeneous rectangular elastic membrane.