Hello libove,
its a very old question, and maybe you already have an answer. However, the webinterface of this printer is truly horrible, slow, and doesn't provide adequate error messages if something goes wrong. So maybe this helps somebody else who struggles.
This is how it worked for me to sign the CSR with our own CA using XCA (but any other tools should be just fine)
1) make sure you first import your own CA certificate through Device Management -> CA Certificate Settings. If it doesn't work without a proper error message (could not import, format is wrong, etc), make sure the file extension is pem (I just renamed my ca.crt to ca.pem and it worked)
2) generate your CSR, which results in a pem file
3) import the pem file to XCA as a CSR
4) sign it and verify that you choose TLS Server as template, update the validity properly (by default XCA only adds a year but for local network printers, often longer periods are fine), and key usage has to be minimum Digital Signature, Key Encipherment, Key Agreement (all that should be added by default)
5) export in DER format as a file with cer file extension
6) import the cer file to the Remote UI. if you get an error message, make sure that the filename is only alphanumeric characters and only 8 characters wrong e.g. "cert.cer" (otherwise you get again some non-descriptive error message and it took me some time to find out its just the filename length that is not OK)
7) if the import is successful, you can use this certificate for the Remote UI: network settings -> TLS settings -> Key and certificate ...
😎 lastly enable TLS in license/other -> Remote UI ....
9) shutdown, wait 10 seconds, start the printer again.
