Look over the upgrade video, especially at disassembly at the beginning and reassembly part at the end - and check out here the wifi antenna is put into. It may have been that the previous owner tried to upgrade it before (more RAM) and then just stuffed the antenna back willy-nilly so that the signal gets blocked by the metal casing and results in a weak signal that the Mini will ignore or forget. Putting back the antenna where it belongs should fix it.