Dear Judith
If you have "clamped" your road bike and the rear wheel is in contact with the roller (the bars of the Elite Qubo Fluid have been lowered to the rear as far as possible), you should be able to shift gears normally - just like when riding on the road. Shifting gears has no influence on the functionality of the roller trainer, only on the pedalling resistance.
If by your question you mean that all "higher" gears feel too hard for you, it may have to do with the fact that you have no pedalling breaks and no cooling breeze on the roller. In other words, exercising on the roller is a lot more strenuous than on the road.

Best regards,
Martin

P.S.
Have you bought an elevation for your front wheel? This makes training on the roller much more comfortable.