The fact that you're using G55 is irrelevent - G55 has no relation to tool diameter offsets. To apply tool diameter offsets, you must have G40/G41/G42 in your G-code. It is not a "global" setting that you can turn on and off with a button like length offsets - it has to be applied to specific moves in the G-code, and there are a number of rules about how it must be used. You'll need to find a book or online article that explains how to use it - it's non-trivial to use it correctly. Better still, use CAM to generate your G-code, and let the CAM software do the heavy lifting for you.
Regards,
Ray L.