So
1) you need to fetch the name of the brand, ie ford or bmw and you currently do this with $brand = get_field('brand');
2) you then need to fetch the models using this brand variable and your naming structure is model_$brand essentially. to do this you’d then say $models = get_field('model_'.$brand);
Have you tried this?
What do you see if you try do print_r($models) ?
