Now that I think about it again, didn't Law saw he's never been to Dressrosa? I don't see any reason for him to lie to the SHs about such a thing and it'd be strange for him never to have visited the place if that was the "base of operations"
I'm assuming he worked for Dofla in between that 10 and 20 year period; maybe there was some sort of period of unrest on the island and Dofu restored it 10 years ago @__@? Idk
Law was like Bellamy in that he was a rookie pirate who worked under Doflamingo's flag, but that doesn't mean they had to go to Dressrosa in order to comunicate with Doflamingo. After all, Dofla has a whole network of underground communication at his disposal and he seems to have no problem at travelling seeing he can apparently walk through the sky.
Dofla probably met Law at North Blue and offered him a chance to work under his flag. Bellamy was also from North Blue. In fact, I think Oda did mention once that Smilies Jolly Rogers are popular in North Blue, implying Doflamingo has a lot of influence over there.