So if I understand you and the drawing correctly you have 3 QNAP NAS boxes in total?
One of those seems to be a datastore for VMs on your ESXi host, the other one is your backup target and the third one is currently unused? Or is the third one currently used for both these tasks and you want to separate backup and datastore?
However the case, these are tasks that are not directly connected with each other so allow me to tackle them one-by-one:
First, your backup target:
Ideally you want to configure a NAS share on your QNAP (either SMB or NFS, me personally I prefer SMB for VEEAM). This is done by having a volume and then creating a share in it. Ideally you want to set the permission of this share so that only the backup user configured in Veeam has access to it and no one else (otherwise someone could just connect to the share and delete the backups).
Then you can add a backup repository on your Veeam server that points to the SMB share on your QNAP. If you configure a backup in Veeam you would then just chose this repository as a backup target. If you add your ESXi host to Veeam it doesn't matter where your VM is, as Veeam can backup the VM directly from the ESXi host.
The datastore is similar. Instead of a share though you want to create a iSCSI LUN (ideally block based if possible). You will want to add the iSCSI iqn of your ESXi host to the QNAP so that the host has permission to access the LUN. You can see that on your host under storage adapters (you might need to add a software adapter if you don't have one yet). On the ESXi host you will then need to add the iSCSI IP Adress of your QNAP, dynamic discovery should work, if not you need to add the iqn of the qnap under static discovery. If everything has been done correctly you can click rescan storage, rescan adapter and you should be able to create a datastore on the new iSCSI LUN just added. This LUN will be listed under storage devices on the ESXi host.
As this appears to just be a LAB it should be fine, however if this were production there are some things not ideal here. First, you talk about DC1 and DC2. I assume these are Active Directory Domain Controllers. If this is the case you should not install Veeam B&R on a Domain Controller directly. You should run this on a separate VM which ideally isn't domain joined (in order to avoid someone logging in to the backup server who shouldn't).
Secondly you would probably not want to backup the backup server with Veeam itself. You only need to do a Veeam Config Backup (configured in the settings) on a target and should you lose your Veeam server just import this data on the new server.
I hope this answered your question and you are now armed to find the additional resources to get this done.