Are you using your right hand to rotate the rear derailleur (specifically the top pulley) rearward as you aim the bike over the trainer axle mount?
You need to rotate the RD body backwards to get the pulley cleanly past the cassette cogs.
- I place my thumb at the rear of the RD (just behind the main pivot at the back,
- Then index and middle fingers on the top of the RD swing links,
- and pull the links down/back in the arc along the pivot at the thumb.
This “opens” things up to allow for easier passing of the RD down below the cassette. Once you have the RD down and the bike on the “hub” support, you can release the RD.
Please excuse the terrible pic, but it may help show what I describe above:
