Just because your new dog is old enough that he should know where to potty doesn’t mean that he actually does. An adult dog has the necessary muscle control to hold it for longer periods of time, but if no one ever taught him the potty training rules then he’s the equivalent of a brand new puppy. Age should not be equated with housetraining prowess, so it’s a good idea to treat any dog new to your home as if he’s still learning the potty training ropes.
Seeking professional help doesn't mean ditching your DIY training program. You can find a professional dog trainer who offers private training sessions, and some trainers even offer online sessions. Many dog owners prefer to join a local dog obedience class so they will be under the supervision of a dog training instructor without the higher cost of private sessions. Plus, classes challenge your dog to learn around the distractions of other dogs.
The process is the exactly same for older dogs as it is for puppies; prevent accidents from happening by using baby gates and a crate if possible, create a predictable schedule, learn to recognize your dog’s potty signals, accompany your dog outside and immediately reward for all outdoor elimination, properly clean any accidents and supervise your dog until you’re sure that he understands the rules.