Webmin certificate issue

Hi all,

I have a Webmin running on Ubuntu which is also running UBNT Virtual Wireless LAN Controller for several sites. I have installed a certificate for the controller and this is working fine. My colleague insisted on installing Webmin for easy server management. I'd like to secure connection to the server over https with the proper certificate. When I apply existing certificate I've used for ubnt (the FQDN is the same, just different port) I get an error as below. I installed the certificate in Webmin\Webmin Configuration.

fqdn uses an unsupported protocol. ERR_SSL_VERSION_OR_CIPHER_MISMATCH

I have disabled SSL 2.0, SSL 3.0 and TLS1.0, I allow TLS 1.1 and 1.2. I believe this is due to the .pem file with the private key which I did not generate initially when created csr for out certificate. It wasn't needed before but perhaps I'm wrong and the problem is elsewhere.

Can anyone guide me if the different certificate is needed or if there is any other method to secure the connection? I know about LetsEncrypt but unless renewal can be automated I don't want to go down this route.

Thanks