I'm with all those who use the word "No".
I've raised four puppies into happy,
well-balanced, well-trained dogs and I have never been bitten on the ankles. Not once. Why? Because I am firmly of the opinion that a puppy (or adult dog) should not be allowed to puppy bite or mouthe a human being. I would have delivered a sharp "No" the first time it happened, followed by lots of praise for stopping. I am incredulous that you have been advised to stand and freeze. Puppies are not mind readers. How is the puppy meant to know that biting ankles is wrong?
I would argue that the word "No" - delivered in a short, harsh tone and meaning "stop what you are doing immediately" - is the most important command a puppy can learn. It is quickly balanced out by generous praise when the puppy or adult dog obeys this command.
Furthermore, allowing any type of behaviour without a verbal correction is tantamount to giving your permission to it. Unfortunately, your puppy now thinks this is acceptable. However, he is only 12 weeks old and I would start today with "No". You'll need to be persistent and have everyone in the household doing the same thing.
I would also, as I've done with all my pups from week one, teach him to fetch and retrieve and spend a couple of brief periods a day (10 mins) playing this game. He may not retrieve immediately, that's fine, but the action of chasing and picking up a Kong, ball or ragger will keep his mouth and his brain busy. Puppies have a lot of mental energy and this will give him a "job" to do, in addition to his obedience training.
Looking back over the lives of my dogs including my remaining dog, obeying the word "No" has saved my dogs on several occasions from potential danger. On our country walks over the years we've come across an escaped goat, an escaped Shetland pony and a very aggressive feral cat. My dogs have never been aggressive towards other animals but "No" in these instances, meaning "do not approach", followed by the recall command, kept them safe.
Good luck!