I'm afraid then you'll have to rely on other apps or study materials. With the sheer amount of data Renshuu uses, offline mode isn't really doable, at least for now. For more information, go to Community > Forums > Search > and look for "offline" ticking the box for thread names. One of the results is even sticky-ed: https://app.renshuu.org/forums...
Iirc I think the crosswords by renshuu app is offline. For studying kanji, you can just download or print out kanji writing sheets. For vocab, make double sided flashcards with one side having a word written on it and the other having the definition written down. You can also make kanji flashcards in a similar fashion (Kanji on the front, definition, readings, radical, etc. on the back). For grammar... I think maybe buying a workbook could be good.
