I have a Galaxy S4, SGH-I337M. I installed an unofficial CM 12.1 ROM because they've only released official CM ROMs up to 11.0. I brought it into a store to get it network-unlocked from Telus Mobility, and they told me that since I installed CM, they'd have trouble finding a working unlock code. Will they be able to find one eventually (they said give them 24 hours to find one) or should I go back to a stock TouchWiz ROM temporarily?
It is better to install your stock rom and recovery because your phone will be originaly supported by the seller. Now they will probably contact CM somehow and this will cost you more time and money.