I just brought to my 3 cat household a 14 week old Westie.

The puppy is either in a pen, on a leash or fully supervised by me.
While in the pen, the cats can watch her and get sued to her at their own pace. Over the course of 4 days they have gotten closer and closer to the pen and feel muchn less threatened.

While not in the pen, she is on a leash with me inside the house. Over the last few days the cats have realsied that she is not free to chase them and so they feel safer and safer and come closer of their own accord.

I take her into the back yard for exercise and little training session....always on the kleash and now, on our 5th day its really sweet because all 3 cats are watching her...they stand just out of reach of her retractable leash.

The ragdoll cat got really curious and came close. The puppy chased her. The ragdoll turned around, faced the puppy and slapped her good and proper. The puppy cried and I was concerned, but she was unharmed. Now the puppy thinks twice before thinking about chasing the cat and she changes her mind.
The other two cats keep running. That's ok. I will give them time to get used to her.....on their own terms.

What I would suggest (and I'm not sure if it would work with an adult dog but this is my best guess) is that you put your dog on an intensive training plan for say 3 -4 days. In that time, keep him under control with a leash (I use a retractable type one that I can let go longer or bring back shorter as required.

Work out a punishment - negative reinforcement. This might be a NO and then a timeout for 5 minutes in the laundry or toilet (as an example)

Keep your dog under the control of this leash at all times. When he approaches a cat gently thats ok, but if he goes to chases, do the punishment. When he is gentle, reward him with food (I use dried up liver bits that come prepackaged).

While you are at work or shopping, during these days keep the dog away from the cats....somehow....in a crate, the laundry, at the neighbours house or something.

The only other thing I can add is that I think this is totally solvable. You dog CAN learn to respect the cats. If your efforts do not work, I would get in a professional trainer. I once used a trainer for 1 hour. Cost me $180. Was the best money I ever spent in my whole entire life. These people are good - its what they do. I have had dogs and cats all my life. I am more of a cat person. But I would never bring them up and keep them separate. They must learn to be together peacefully. To have the situation in any other way, is to place both at risk. a Dog can injure or kill a cat, and a cat can certainly cause major injuries to a dog as well.

Good luck! Don't give up. You can do this!

Note to add: My other dog sleeps with the cats. They groom each other. They respect each other and they are friends. My previous dog that I lost to cancer was exactly the same. These animals would never hurt each other...not even play rough.