Here is a very basic indexing guide.
If you’re struggling going from smaller sprockets to bigger ones (going in the direction of 11t to 28t) looking from the back of the trainer turn the barrel adjuster on your rear mech anti-clockwise. Just do a quarter of a turn at a time. Remember how many you do so you can always put it back if it gets worse.
If you have trouble changing from big to small turn the adjuster clockwise but still a small amount at a time.
If the middle of the cassette is fine but the two extremes are bad then you need to adjust the limit screws. There will be 2 one marked H which changes how far to the right the mech can go so adjust this if the 11t is a problem. Again a tiny bit at a time.
The other is marked L and is for the inner limit (28t). Be very careful with this, you don’t want your mech going into the spokes when back on the bike. Adjust this if you can’t get into the biggest sprocket.
Plenty of videos online showing how to do it.
It is a very useful skill to have.
Just remember what you have done so you can always put it back to how it was if things get worse.