Max, I make it an answer to your question.
Which computers you can reach configured on a network or cannot reach, cannot be overridden by software on a computer. That would be a security-breach, and it could be a cause for address conflicts, because in different NAT-areaś the same IP addresses can be in use.
So, You cannot see computers behind a router, if that router uses NAT.
The router does not advertise these addresses.
The purpose of NAT is to have a special island of IP-addresses which no-one outside the NAT section can see. In this way, a company can use fewer unique IP-addresses to have a good functioning network. Another purpose is security. The router also hides the MAC-addresses of the computers inside the NAT area. A router can hide many things.
Computers inside a NAT-area can initiate contact to computers outside the NAT ((if permitted) the router will remember the computer and keep a address-translation for it), but there is no way a computer outside the NAT can address a computer inside the NAT. It can only reply to a computer from inside. It does that by replying to the router, and the router will know to which computer in the NAT area to forward the reply.
Go does not provide libraries which can solve these external limitations. Live with it. There is nothing you can do.
As JBuchel explains the skype model can work if there are some extra provisions, like open UDP ports, and extra server/computer, etc. This is in fact a rearrangement of the network configuration, which cannot be done without the help of system/network administrators.
But if there is support on that level, the solution is so much easier, just remove the computer from the NAT and give it an IP address that is visible for the other computer.
