Have the same problem. Cycleops Hammer and IOS. Logged a ticket and got this reply:
hanks for contacting us at Zwift Support!
I’m sorry that ERG mode isn’t working on your trainer in Zwift.
I’m sorry to say, but there’s a recent known issue of lack of resistance in ERG mode for some smart trainers. Our team is looking into this for a fix as efficiently as possible, as we’re sorry for the frustration this may have caused.
In the meantime, it can help to check that the firmware on your trainer is updated, and that you’re not experiencing Bluetooth interference.
We have discovered this is a bug, if the steps above do not help, we are currently working on a fix, so make sure to keep your game updated! We released an update for Mac last night, so an update for other platforms is coming soon! (Not change for AppleTv or iPad for me so far though)
I hope this information helps shed light on the situation. You’re more than welcome to reach back out to us if you have any other questions.