Arghhhhhhh!!!! Why did I try to get to the bottom of S/MIME certificates?!?!

The process should be smoother and more straighforward than GPG, but it’s taking way way longer than the time I have available for it! GPG took 10 minutes, this is taking hours!

Got the damn certificate, installed with no problems on macOS, it’s working as it should. On iOS, on the other hand, I can’t get it to work. Tried the normal way, as instructed, but fails to sign messages. Tried a whole lot of variations, still not working. Searched online and tried again, nope. Deleted main mail account from phone, installed again, certficate all over, still nothing. About to give up, it’s late and I should be dealing with the photos I have to edit, not another thing that will serve close to no purpose!