Trains start from Luxembourg at 05:22, arriving 05:58...presume this is too early for you, so suggest:
6:06 arriving 6:44
6:21 arriving 6:59
6:36 arriving 7:14
Given the walk from RER to 2B (a good ten minutes), and allowing sufficient time for check-in, I'd opt for the 6:06... allows extra time for spotting, too!