It was OpenVPN with a TOTP 6 digit google authenticator MFA code. It worked just fine with Tunnelblick in MacOS and Viscosity in Windows, but I never found a setup that worked in Linux. And I searched for a while. I had to switch away from linux because there was no good solution to this.
And it's not the only non-easily fixed issue I've had with Linux over the years. The classic saying is "Linux is only free if you don't value your time" and I've found that to be true.
