We can recommend you to travel via the airports in Leipzig/Halle (LEJ), Nuernberg (NUE), Berlin (BER) or Frankfurt/Main (FRA), from there are convenient train connections (ICE) via Erfurt to Ilmenau. From Leipzig/Halle to Erfurt you need about 1.5 hours by train, from Nuernberg about 2 hours, from Berlin about 2.5 hours and from Frankfurt about 3 hours.
From Erfurt you can use connecting trains to Ilmenau. The travel time for this is about 1 hour. Of course, you can also cover the remaining approx. 45 km by car or cab. In this case, we recommend that you use the A71 federal highway and exit 15 "Ilmenau-Ost".