He's an Australian, and so am I. His ISP (Optus) doesn't go out their way to block ports. However, he says he's using a AC800s 4G/LTE modem and backs that up

He's an Australian, and so am I. His ISP (Optus) doesn't go out their way to block ports. However, he says he's using a AC800s 4G/LTE modem and backs that up by saying is using mobile data. The default mobile data service from all telco's will use CGNAT and will hand him some 10.0.0.0 address. I presume that's because mobile phones don't run servers as a rule, so no one notices and it saves them a bit of money. They don't deliberately block any particular port, but CGNAT effectively blocks all of them. He could pay a extra and get mobile data with a static IP Address if he wished. Not from Optus - they discontinued that service (which gives an indication of popular it is), but other virtual carriers based on Optus still offer it. I guess it's not worth the money to him. Everything supports IPv6 now, and IPv6 happily carries IPv4. Why the telco's are still dicking around with CGNAT and IPv4 is a bit of a mystery.