HTC One M8 AT&T - Unlocked (Update Not Working)

I have HTC One M8 (AT&T - Unlocked - GSM Unlocked)

So far I have recovery TWRP/bootloader.. nothing else works I cant boot my phone.. Kind of lost what to do .. I really want to install a custom ROM like Skydragon.. I got really close
Unlocked phone (S-On), rooted. Flashed bootloader and installed skydragon, but OS wouldn't boot up im guessing cause I need to flash android to 5.1? My phone has 4.4.5. What must I do to get this working. Must I flash bootloader or kernal or android to latest??? Any help would be great!!!

Thanks!