It does sound like from your questions that you've done some reading and know something about housebreaking - so first - pat yourself on the back.
OK - now to begin with - here's some bad news:
Where did you get your Puggle? Many people who purchase these mixed breeds - especially if purchased from the internet or a pet shop are unaware that the vast majority of these designer mixed breed dogs are coming from puppy mills (there's even an Amish website devoted specifically to Puggles - and beleive me - I have NO intention of giving them a link here). Among the wide variety of problems with purchasing a dog from one of these establishments is that these dogs spend their entire puppyhood in a kennel - sometimes a wire pen not much bigger or wider than can contain a mother and her pups. While dog instinctively try to keep their dens clean (and thus the premise behind housebreaking), dogs who are born in the conditions I have just described cannot keep their dens clean and wind up developing the habit of going where they sleep. This is compounded in the pet shop, where again, puppies are kept in cages 24/7. So if your puppy comes from either of these situations, and not from a private breeder, you are in for an uphill battle and crate training may not work for you.
Now, you may also have had some backsliding because your dog has come out of surgery. He may have one accident in his kennel because he had a slight loose stool from the anestisia from his neutering. ONce his kennel smells like poo, he will start to go back there and use it as a latrine. Make sure you wash down his crate with an enzymatic cleaner like "Get Serious!" or "Nature's Miracle" and let it SIT for at least five minutes. Wash all bedding as well to get the scent out.
Next, are you regulating water and food intake and keeping a chart? You want to chart both the times he is going and the times he is drinking and eating. Try tweaking his schedule. If he is making mistakes in his crate at around the same time each day - that is what I call his "witching hour." Try to change your own schedule around to get him out of the house at that time and do not let him back into the house until he has releive himself (yes - that means you stay out too!). The key here is consistency!
Another thing - which may not be the root of the problem but could be adding to the problem - IAMS is a corn based dog food. If you look at the ingrediants, while corn is not the first ingrediant, you will see corn or corn meal in the ingrediant list. That is adding bulk to your dogs stool and may even be causing him to go to the bathroom more often. Try switching to an ultra premium brand with no fillers or corn or corn by products or corn meal. Your dogs bathroom needs may drop significantly. Whole Dog Journal prints an annual dog food review -most of the foods they recommend are top quality and none are corn based.
whole dog journal