Train advice

Need more detail! Have you checked on the Trenitalia website?

There's not a Freccia line between those areas as far as I know, so the trains won't be particularly fast.

For a better answer, tell us your origin and destination.

We plan to arrive in Milan and take a month to get to Calabria. No Rome on this trip.

We meant your origin in Puglia and destination in Calabria. but you have a high level plan to sort out.
With a month to play with--even though it will be filled in fast--I would not worry too much about getting between those places. But I would want a car for both Puglia and Calabria.
Do you then need to make your way back to Milan? I always work out an itinerary by writing it out day by day.

From where to where exactly? There are many stations in both regions.
Generally I'd say it is a rather long trip. Most trains from Puglia to Calabria would pass through Taranto and then run along the Ionian coast to Calabria. They will certainly require one or more connections unless you depart from Taranto (Puglia), which has trains that go all the way to Reggio Calabria (along the Ionian Coast). From Taranto to Reggio Calabria (no changes required) it's over 6 hours (it's a one track railway, for the most part).