Help upgrading OS on rooted S4

[dzegers]

Sorry if this topic has already been discussed but I could not find it for my specific model. I just got a 2nd hand S4 SGH-I337M, 4.4.2. It is unlocked and rooted. Originally on Bell and i am using on Rogers. I want to install Lollipop 5.0 on it but i get the message that the phone has been modified and wont update by itself. I downloaded root checker to confirm root and it said it was. I used Supersu to unroot in hopes that it would allow me to update OTA but did not work. Is there a way I can re-root and then be able to some how install Lollipop. Thanks in advance for any help.