User Profile

First, get the certificate onto your iPhone or iPad. Airdrop the .crt file, or email it to yourself. Then open it on the device, and new in iOS 13 you get a notification that you have to enable it in a separate step from Settings.In Settings, find the certificate (it showed in a separate temporary category at the top of Settings, but should also be under Settings->General->Profiles) and choose to install it. There are several accept steps. Check at each step that you're installing what you think you are.Finally, go back to Settings->General->About->Certificate Trust Settings, and enable the self-signed Root cerficate in that panel. See https://support.apple.com/en-nz/HT204477 for more details.As with the OP, I found that this only worked for newly-added mail accounts. I had to remove and re-add my server to get it to accept the self-signed root.