Update: This issue should be fixed as of build 1.0.25226
We're currently investigating an issue where SIM mode doesn't engage on the Wattbike Atom. So far this has been confirmed on ANT+ FE-C.
If you're affected by this issue, you may want to try connecting with a different protocol instead. We've received reports that the Bluetooth connection over the Zwift Companion app still changes resistance. More information on how to pair via ZC is on the knowledge base.