do you use the UCS self-signed certificate or did you install Letsencrypt. With a vilid Letxencrypt certificate it should work.
Using self-signed cetificate on IOS 10 and above is a little bit tricky.
One quick and dirty method is downloading the root certificate and the SSL certificate onto the iPhone. After that go to Settings - General - Info/About - Certificate Trust Settings, and there is a section called
“ENABLE FULL TRUST FOR ROOT CERTIFICATES”.
Under it lists the certificate(s), you installed previously on your iPhone. Enable them.
After this corcedure you should be able to setup the Kopano account on your iPhone.
I use this method on our company-Exchange Server since 6 years and it worked all the time on alle Apple devices as long as te certificates are not outdated.
It is a poor way to use and so I will switch to Letsencrypt next.
I recommend, first to install and configure Letsencrypt with your Kopano and connect to your Kopano account then.