Hello,
I'd like to share that the IP address mentioned in Developer forum post 17.188.143.34:443 is not the one which responds to api.sandbox.push.apple.com:443 address.
I found out the hard way so I am sharing it with others.
The actual IP address can be found using 3rd-party services which show the information about server's SSL certificate like sslshopper[dot]com. I'll put the one I discovered using this method here for convenience:
17.188.143.66
I hope the official post will get updated though 🤞🏻