Shifting problems

I have a BMC Road Machine 01 3, 11 speed bike. Someone bought me The TacX Flux S trainer as a gift. Before i just sell it which will crush them, i want to see if I can try anything to get it to work.

I have never been able to get the gears to shift properly. I even took my cassette off my bike which shifts perfect and put it on the trainer.

The gears never stay in place and sometimes just jump to the top of the cassette or off it and in back of the gears.

I am using the T2875-01 with 11 speed cassette the proper adapter on the unit.

It never stays in gear.

Has anyone had this issue and if so how did you solve it?

The bike shifts perfectly on the road.

So frustrated.

I have contacted Support and they try to help but so far no solution.

Thanks in advance