How to restore to stock ROM on international version to receive OTA updates

A while back, I rooted my HTC One M8 and loaded TWRP in an attempt to turn it into a GPE phone. But, since Firewater was discontinued and SunShine is incompatible with my phone, I couldn't gain S-off and was never able to get Lollipop. Now, I'd like to unroot my phone to receive the OTA update of Lollipop. I checked the mega RUU/OTA/Backups thread to find a backup with my CID and version-main number but was unable to find one that matched. I also don't have a backup of my own. Is there any way to restore my M8 to stock factory settings and get Lollipop?

Relevant info:
CID#: HTC__621
version-main: 3.28.709.6
Android version: 4.4.4