A359 not following VNAV (read the manual)

Hello there,
I am having issues with the A350. I am very used to the A320, in fact, I am a real pilot on the A320, but in the simulator, with the A350, the managed indication light is not present like on the A320, however, in the A350 you have an ALT (BUTTON) below the altitude window in the FCU, and that is confusing, as my aircraft WILL not be managed and will not follow VERTICAL NAVIGATION and or altitude constrains. For example, when I am passing the TOD, I get a message I have passed the TOD and I should start descending...in the A320, it will descend automatically when managed, following VNAV once altitude is set to a lower altitude that you want to descend...but the A350 will NOT DO THAT unless I manually select it. Any help is appreciated, as the above did not really answer this question. How do I know it is MANAGED on the A350?

Thanks a lot

Cheers