Can't Use External IP On Hetzner VPS
Answer : Hetzner had stopped assigning public IPv4 addresses to virtual servers. As far as I can tell this change happened when they changed the product name from VQ to CX. The usage of NAT is not mentioned in the product description though. Eventually Hetzner introduced a newer cloud platform in which VMs get real public IPv4 addresses and a routed /64 IPv6 prefix. Both versions share the CX name. Virtual servers ordered in 2012 and 2013 would keep their public IPv4 address until 2019 when the VQ line was discontinued. But virtual servers ordered in 2016 only have an RFC1918 address, and Hetzner will not route a public IPv4 address to such a virtual server. They still allocated a dedicated public IPv4 address to each virtual server, which they NAT to the assigned RFC1918 address. Hetzner believed this was not a problem because it was a 1:1 NAT. As you found out, this is error prone when configuring the server. You have to know about this NAT. And you have to look up the map...