Also, are you on stock? I had 4g LTE on stock, and LOS 14.1, but on OMNIROM 8.1 I only have 3G which could just mean it's APN issues. Maybe switch to a more configurable ROM like LOS 14.1, and edit the APN settings? Rooting Omni atm to see if I can edit those apn settings.
edit: rooted and...