Zyxel 326 NAS out of box has iSCSI support, but only with limitations, you have to fill all security parameters and you have to restrict that the lun/volume is only accessible from one iscsi client. This means impossible to connect from two ESXi hypervisor to the same Zyxel 326 nas.

If you install Debian to Zyxel 326 NAS (
http://modernhackers.com/install-debian-linux-into-zyxel-326-nas/ ), you will have the chance to install custom iSCSI setup

Follow to steps to have iSCSI configured for ESXi use on Zyxel 326:

apt-get instal linstall tgt dkms

vi /etc/tgt/conf.d/target01.conf

cat /etc/tgt/conf.d/target01.conf

<target iqn.2019-10.com.modernhackers:server.target01>
    backing-store /dev/md0
</target>

service tgt restart

Then after you can configure your ESXi Servers to access the NAS on iSCSI

tgtadm --mode target --op show
Target 1: iqn.2019-10.com.modernhackers:server.target01
    System information:
        Driver: iscsi
        State: ready
    I_T nexus information:
        I_T nexus: 1
            Initiator: iqn.1998-01.com.vmware:server1-61d7602f alias: none
            Connection: 0
                IP Address: 192.168.2.21
        I_T nexus: 2
            Initiator: iqn.1998-01.com.vmware:server2-0a9062f7 alias: none
            Connection: 0
                IP Address: 192.168.2.22
    LUN information:
        LUN: 0
            Type: controller
            SCSI ID: IET     00010000
            SCSI SN: beaf10
            Size: 0 MB, Block size: 1
            Online: Yes
            Removable media: No
            Prevent removal: No
            Readonly: No
            SWP: No
            Thin-provisioning: No
            Backing store type: null
            Backing store path: None
            Backing store flags:
        LUN: 1
            Type: disk
            SCSI ID: IET     00010001
            SCSI SN: beaf11
            Size: 1000070 MB, Block size: 512
            Online: Yes
            Removable media: No
            Prevent removal: No
            Readonly: No
            SWP: No
            Thin-provisioning: No
            Backing store type: rdwr
            Backing store path: /dev/md0
            Backing store flags:
    Account information:
    ACL information:
        ALL