Why are you buying just stem? I've never heard of anyone doing that & am skeptical of someone selling that way. Who's to say (1) it's what they say it is, OR (2) that it'll grow?

Maybe also post this at the Aroid Forum & ask them if this sounds legit?

Of course, I could be wrong, but I'm wondering if you've just been had.

Since I'm sure it was expensive, I would definitely do some research. You could start by asking the seller for advice. And I know there are some great youtube videos about growing philodendron from a stem cutting. I have seen some that recommend using moistened sphaghnum moss for a while in like a little clear box to retain some humidity but not too much because you don't want it to rot.

Legends of Monstera on youtube has a two-part series on propagating that has some good info in it.

Best of luck!