Interpersonal (Hamburg) and DLR (Hamburg) are two different challenges
DLR = "Astronaut" tests, very demanding, takes weeks of prep
Interpersonal = tests on CUT-E level, very doable, passing grade depends on the airline's individual requirements, forwarded to Interpersonal. Prep for a few days and you are good, provided your English is solid and you remember something from your ATPL. They might, e,g., tell you that your tests was good for Condor but insufficient for DHL or vice versa. You can take one test and re-use it for different airlines, that is also a plus