There are a lot of free VPN services out there, but if you're looking for something truly undetectable, VpnHood is the only option. It's open-source, and the code is available on GitHub, so it can't be blocked by any firewall.

VpnHood application allows users to access the internet securely and privately by encrypting their internet traffic and routing it through remote servers. It also offers a few free public servers that change the IPs every now and then, so you can create or order a private server if you need it to be completely untraceable and safe.

The service offers split tunneling by apps and countries as well as Android TV support—and it uses IPv6!