Hey there. While I have NO suggestions for *fixing* the issue on Sierra -- and I'm not running Sierra on my MBP, and won't for a while -- the diagnostic route is pretty clear. Try other stuff! First thing is to attempt with an ad-hoc network. Try recreating the Network connections too, perhaps using different ports. Try connecting backwards (initiating the connect from the computer). If you can borrow or beg an iPhone or a later iPad, test with that too. If you have another router to try, do that. Etc. etc. The changes that Apple made to networking in iOS10 (not your issue) and Sierra are pretty heavy, and understanding all this IPv4 and IPv6 stuff is well beyond my paygrade... but black-box testing leads to surprising and often positive results. Please do report back.
Thanks!
Dan (Author, MD)