In this case it may be the best idea to use a VPN, but i guess if so many services from other countries are blocked that most VPN providers are blocked too. So i may be the easiest to setup a Wireguard GW on the german VPS and connect the ISPConfig Server to it as a client. If VPN works, may consider a privacy focused one like Mullvad, they are accepting crypto and cash too for more privacy. More details on the process: https://www.linuxandubuntu.com/home/setup-vpn-with-wireguard-vpn
The VPN system in Iran is like the China VPS system. It blocks IPs which has more traffic outside of Iran. VPS is the safe and easy way to me if could configure one of these mentioned systems. What you recommend is very nice and I'm working on how to implement it on both Debian, I mean German Debian, and my Iran Debinans. shuttle & WG looks very powerful.
You are right. I may have to do this also. I have a server configured 8 years ago and another 3 years ago and also one brand-new server RAID and ready for installation. In that way I must have lots of CDs/DVDs, it is ok but sometimes you stock lot's of hours to solve a simple solution. That's why I am looking for more robust and stable ways.