You say that you have to be connected to the internet to unlock achievements. If you unlock one offline does connecting to the internet and reopening the game later update it so the achievement appears unlocked on your Xbox.com?
Challenge/Franchise: Tier One , Tier Two , Tier Three , Tier Four  | Staff:  | Subscriber: 
Overachiever:  | Arcade Crazy: 
Retail:  | Arcade:  | WP:  | PC:  | XB1: