Agree somewhat with rickms80. You might try a factory reset to try and kick the phone into gear for OTA updates.
However, LRX210 is not a prerelease early OS build. It's an OTA update build for early Nexus 6, pre-installed on later ones. When you started setting up the phone it should have automatically started updating Android in stages. Probably would have stopped at Android 5.1.1 but it's possible it would go all the way to 6.0 Marshmallow.
I don't have a reason why it would fail to start updating the OS. It should have updated with or without a SIM. Having an activated SIM in the phone would cause it to load a CD carrier-specific build of Android if needed and available, like the T-Mobile build.