Cat tray + dog = dog eats cat poo.

Nothing you can do.

Except put the tray where cat has access and dog never has. That is the only way.

When dog has something in mouth and refuses to give it and runs away, no matter how disgusting, you give it one chance to exchange for a treat and then you leave it.
If your dog runs into the garden - even better, shut the door and leave it there for ten minutes. Think of an excuse to go into the garden, hang out a towel, go to the shed for a look...anything except the dog. Don't look at dog, don't talk to dog. If the dog does not take the opportunity to get inside it is because it thinks you will play in the end. Close the door again and wait fifteen minutes before you HAVE to do something in the garden. Check if the towel is dry yet, see if the shed has been hiding what you were looking for before. Carry this on until the dog comes in on it's own. Do not look through the window inbetween times.

The other thing that might be useful is playing with a ball or rag toy, not tug of war. The trick is....you actually have two of the toy!! You throw one - the dog runs to get it - the dog tries to get you to chase it - but you are busy playing with your own and are ignoring pup!!! You only throw the toy you play with after you pick up the pup's toy.

Sorry but training the cat to look for the tray in a different place or behind a barrier that it can jump is going to be the most successful option. My dogs' have always left it on command....even left it alone for several weeks.....but it is a natural urge and it is better avoided than challenged.

Finally, cats and dogs in the same household should be wormed at the same time if at all possible. (Ask your vet to sychronise things.) Dogs, and especially puppies are likely to pick up worms from cat faeces, cat fleas and it works the other way round too.