He's just starting to teeth and that means he wants to chew anything he can.
What I have found best in working with pups that are at the chewing stage is a combination of things. Good chew toys like you are using, when I catch the pup chewing something it shouldn't then I tell it no plus I give it a 'legal' chew item and praise the pup for having that instead. The praise part is as important as using No is! I also use product called Bitter Apple to spray on things I don't want the pup to chew - most pet supply stores carry it. Other products include Sour Grapes, Bitter Orange etc.
If you cannot watch your puppy set up a puppy proofed space you can safely leave him or use a crate to confine him and give him a chew toy to keep him busy. If he does it by sneaking away while you are watching him you can tether him to you with his leash so he can't sneak off giving you a better chance to correct him, give him a good toy, and praise him for having it.
Hope this helps you!
Please press Accept if you find my answer helpful. Want more info? Just reply on your question!
Bonuses for good work are always appreciated!
Thanks for the opportunity to help you!