Hi all the installations hosted by 3CX must use a 3CX FQDN. Custom domains are not allowed.
If you plan to use a custom domain then you need to do it as On Premise or Self Hosted.
If you are ok using a 3CX FQDN then take a backup without the license key information and fqdn, save the backup and uninstall the server. Once done, access to the customer portal with the email registered in that license key and disconnect the FQDN. Then you can proceed and install the pbx using Hosted by 3CX and once installed set the backup location to an external server such as ftp, sftp, google storage or sharepoint, place the backup there and restore it from the management console.
Note: check that the backup has a system owner configured with an exclusive email.
Have a nice day!