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Actually the tower/cell transition is a normal behavior of a cellular based node. If the cell transmission requires a handover to continue service then the "move" will take place. There could be a number of reasons for the handoff or handover from one cellular source to another. When the conditions for the threshold is reached where the signal makes the transition it is per how cellular communications work. 

If your GW is covered by both sources and there is a number of variables which change and trigger the handover there is little you can do about that other than to do what is possible to favor one source much better than the other. If say you use a directional antenna, and leverage the shielding properties of your home, if possible, then you might be able to reduce the chance of a handover to the other source. You would need to know the locations fo both cell sources and the signal receive power of each plus consider the distance to each source and the terrain between your gateway and each signal source. The conditions might be such that even with a directional antenna you cannot get the results you desire. Without a bit of research to have a clear knowledge of the differences between the sources you will not know for sure.