USG20-VPN VPN-Wizard issues

We own a USG20-VPN, firmware version v4.33_ABAQ.0, and we need to set up 10 vpn connections through l2tp over ipsec.

We used the "VPN-Wizard" from the "Easy-Mode" to get, immediately, the job done. We set up win10 clients as described here: https://support.zyxel.eu/hc/it/articles/360001390914-Configurazione-L2TP-su-USG-Firewall-utilizzando-il-client-integrato-di-Windows but they do not connect, we even considered/tested the usage of SecuExtender client, but it should not be required, isn't it?

We did stuff this way (for testing):

• all nics were disconnected from lan (wired and wireless)
• only the wifi nic was connected through external connection (5G tethering and then FTTH)
• server-side the config was done first by VPN-Wizard then accordingly to: https://support.zyxel.eu/hc/en-us/articles/360000706899
• the previous link implicitly was referring to Easy-Mode (first part) and Expert-Mode (second part), so the VPN-Wizard in Easy-Mode is not enough...
• then we considered http://onesecurity.zyxel.com/img/uploads/ZyWALL_L2TP_VPN_Setup.pdf from the Expert-Mode panel link for configuring everything, manually, from scratch

In the end the question is:

what is the fastest way to get L2TP/Ipsec up and running?