Iíd recommend you start all over with a new trainer. A puppy can and should be trained with positive reinforcement training and your trainer should be training YOU how to work with your pup.
It doesnít matter how well your pup responds to them if they arenít teaching you how to handle your dog.
The training methods that were considered appropriate even 10-15 years ago have been debunked by science. Tradition isnít a good reason to put a prong on a puppy for example. There may be times when a prong might be a necessary tool (a weaker handler with a large hard dog) but you didn't mention that sort of situation here.
Iím not going to criticize you for not starting training earlier. Iím going to applaud you for recognizing that you needed assistance and seeking it. I just think you found the wrong person. (Old school trainers love to stick prongs on GSDs, rotts, dobes, etc. Maybe that tells us something about their training skills?)
My goal with my pups is that they are so attentive to me that I can whisper commands or just glance in a direction and they comply. I can simply tip my head and my older dogs know what I want. Serious dog handlers - those working with service, therapy, herding, or agility dogs ó donít get loud with their dogs. They whistle, use hand signals or slight body motions.
Loud is not necessary, especially with German Shepherds who have big ears and great hearing. I use mostly a soft voice followed by cheering and clapping when my puppy complies. Training at my house is a party. I recommend it. My goal is 98%+ accuracy with the first cue (command) I start new skills for him to learn every two days. He trains/practices all day long. But we have a great time. My older dogs rush over to show yes! They know that skill too, and they want to be part of the fun.
Iím guessing you prefer that for your puppy?
(Oh, Iím training a working dog. Training is serious business, but ton of fun)
First, get a martingale collar for safety so your dog canít slip out the buckle collar. I like RC Pets martingale collars; preferring chain martingales over webs a little. They last a long time, are sturdy and are comfortable.
Get yourself a pouch for treats. Delicious small soft chewy treats are best.
Check this out:
Do NOT tether a strong dog that does not know how to walk loose leash to you. Thatís a recipe for injury and/or a runaway dog. Never tie one to your clothing, which can easily rip. If you ever want to tether a dog to you, a working dog leash is what we use.
Next, get this book: https://www.dogwise.com/power-of-pos...g-2nd-edition/
Amazon sells it too.
Start reading it ASAP so you understand the sort of training you want to do. Then call local trainers and ask them if they do this sort of training. Look for classes too. (I do both with my pups)
I can recommend books and Videos to you all day, but a *good* local trainer is best.
An inappropriate harsh trainer can ruin a dog. Iíve walked out of classes Iíve already paid for because the trainer was too harsh.
Your pup needs some leadership and guidance, but we can give that while being kind and having fun.
Youíve got this!