B?rbara!!
Of course there are Dominicans in Frankfurt; not as many as in other German cities, but there must be a fair number of them over there. One of the problems is that when you do a Google search starting with the phrase "Dominikaner in (.... here you can put any German city....)", you get results for the Catholic order of Dominican priests.
Go to German-speaking forums about the Dominican Republic and I'm sure you'll know how to go about finding Dominicans in Frankfurt (in diesem Fall am Main, richtig?)