You need to get rid of the the then.
'Now' and 'then' are two advers refering to diffrent times. 'Now' is refering to the present while 'then' is refering to the past. It is not appropriate to use them together. ' Are you online now?' is the right question to ask.
It has no other options. You are mixing things if you say that this is a correct sentence. "Then" is wrong
Being incorrect doesnt make it inacceptabl, otherwise we would all be still thus-ing and thee-ing like ye olde englishe
IMHO, the sentence should have been..Are you online now, then?
It would be correct if there were a comma after now, but it'ds acceptable (at a2 level anyway:))