The ornamental design for a modular potted plantsupportingdevice, as shown and described.
FIG. 1 is a front view of a modular potted plant supporting device, showing my new design;
FIG. 2 is a rear view of the modular potted plant supporting device, a right view and a left view being mirror images thereof;
FIG. 3 is a top view of the modular potted plant supporting device;
FIG. 4 is a bottom view of the modular potted plant supporting device;
FIG. 5 is a top perspective view of the modular potted plant supporting device, the dividing walls in phantom forming no portion of the claimed invention; and,
FIG. 6 is a bottom perspective view of the modular potted plant supporting device.