Don't forward ports you'll only make things worse. Just make sure UPnP is enabled under WAN - Internet Connection.
The error above says that it can't resolve the host name which suggests that DNS isn't working. Have you made any changes that would effect DNS? e.g. LAN or WAN server settings, ad-blocking scripts, VPN client, etc.
Or perhaps it's simply because the Store is offline. In which case try again later.