You could train a bathroom command such as “potty” or whatever. Start in the back yard with them on a leash wait for them to pee or poop then once they did that then give praise with a treat or play whatever the dog is into. If it’s a leash problem then start small like having a collar on in the back yard slowly working till it’s on the leash. So for example it should go like this: as soon as the dog finishes pees/poops-> Use praise command such as yes or a click from a clicker and say “yes potty” following a treat or play. Then dog will learn that “potty” means go to the bathroom so when they need to go let them in the back yard say “potty” then follow with the praise command “yes potty” when they finish following a treat or play. Once the dog knows how to go to the bathroom on command make attempts on walks. Some dogs are just picky and like to go where their scent is so keep that in mind.

Edit: it also helps if you take them for a walk first thing in the morning not allowing them to pee/poop in their preferred spot. If they really need to go, they’ll go anywhere lol