Built-in wifi certificate expired? Any alternatives?

Hey everyone! Last month the built-in wifi certificate expired on my 100E. I'm running the latest possible FortiOS release but without a license. (FortiSwitches and FortiAPs are in place as well)

I'm using the certificate for the builtin WPA3-Enterprise authentication against user groups. Some devices didn't care that it ran out, others had a warning and few right out refused to connect to wifi https://mobdro.bio/ .

Is there a way to get a renewed certificate without a license?

Currently I switched it to a public Let's Encrypt cert, hoping to not get the "untrusted" warning on devices but they still appear and I thing some android devices have issues with the (automatic) renewal of certs, which I'm super duper NOT a fan of, since those devices are business critical.

Any ideas? I'm all ears!