Most of the time i am behind a NAT router either at work or at home and those error are pretty common on my openSUSE system.
The old school trick i always do is ping those hosts.
ping download.opensuse.org
ping downloadcontent.opensuse.org
now edit the file /etc/hosts and add
RESULT-OF-IP-FROM-PING download.opensuse.org
RESULT-OF-IP-FROM-PING downloadcontent.opensuse.org
eg. something like this.
195.135.221.134 download.opensuse.org
195.135.221.157 downloadcontent.opensuse.org
That trick always saves me from choosing a local mirror manually. It works for me and I'm not saying it will work for everyone, just try it and see.
