I'm only asking you this to get it out of the way, so that Elany won't have to ask it: have you used one of your rank unlock tokens to re-unlock the Infantry division after you prestige your soldier?
When you prestige your soldier, everything locks back up, and must be re-unlocked. For the divisions, I believe that you can unlock them at any time, assuming you have a rank unlock token you can spend on it. You should receive a rank unlock token every time your soldier levels up.
Tl;Dr: after you prestige your soldier, everything must be re-unlocked, including the division you wish to use.