Thanks, I have a partial success which involved two steps:

• The wiki says to create a root certifcate with gentls_cert setup with no other parameters; I had to add my domain's data to this command. The new certificate has been downloaded to the phone.
• Replaced the registrar definitions in the phone's config files from IP address to the server's name.
• The above setup worked as-is. To be sure I've added the NAPTR records to the DNS after the above two steps worked.
• BTW, the wiki says that the NAPTR records are not mandatory, thus I did not add them at the first place.