There can be various reasons causing that you do not feel your trainer is simulating the resistance of the road. Most often it is caused by a pairing to the power output of the smart trainer, but sometimes it can be caused by having the ANT+ dongle placed too far away from the trainer:

Paired with power instead of the smart trainer

Some smart trainers also give the option to connect to the power meter (ANT+ Power) of the trainer:

The power meter is not smart and does not simulate the slope of the road. If one wants the smart trainer to simulate the slope of the road, then one should pick ‘smart trainer’ instead. When the smart trainer is picked as input for speed, one will automatically see ‘Open Road’ also pick the smart trainer to control the resistance/grade:

ANT+ signal is too weak

If the ANT+ dongle’s slope signal is very weak, the smart trainer cannot simulate the slope of the road. The smart trainer simply does not have enough data to calculate how much it should brake down the rider. Instead, it does nothing. Even a distance of 50 inches / 1.27 meters between the ANT+ dongle and the smart trainer can cause this problem.

The solution is to put the ANT+ dongle closer to the trainer. This can be done by using a USB extension cable and placing the dongle right next to the trainer.