If the original cable has a DB9 at both ends (as well as DB25) then it is serial. It is a general purpose cable. Originally serial cables were DB25 at both ends for modem signalling, but for a pure data cable only three cores are absolutely necessary and the DB9 got introduced with data and some restricted signalling lines. For CNC to work you must have a 25 core cable with straight through connection on all pins and 25 pin connections at both ends.
[PedantMode] OK 25 not strictly necessary as several are earths which can be common for some applications [/PedantMode].