After half a day of using a certificate (a live stream from a bird feeder at https://birdfeeder.online/) here are some thoughts about the certificate trust level.
In short: most, if not all, desktop browsers accept the certificate without questions. In the same time, most, if not all, mobile browsers complain about untrusted certificate.
I am not sure if this works for everyone but @jpbe helped me out by explaining that for mobile browsers you also need to set the CertificateChain in your apache/nginx (think also nginx at least) config file. I had the same error and now it’s solved