Question about Zyxel USG FLEX 200

i have returned the product. as i bought the wrong one. i also need one that offers wifi.

Hello, I don't know exactly why you want to install another firewall software on a firewall. As far as I know, it is not possible. If you want to use pfSense, you are certainly better off with a freely configurable PC.
The question about the licence is therefore superfluous. But what you should know about licences is that you buy the Zyxel FW once and it runs as a firewall for years. This means that the firewall rule set is not tied to an annual licence. If you want to use services such as the ContentFilter or Web Antivirus and the like, these services are subject to a charge. You can purchase them individually or as a package. The same applies to VPN licences if you want to use the Zyxel VPN client. However, there are of course alternatives for this. You are not tied to the Zyxel VPN client.

I don't know whether pfsense runs on it. I can't mentally put a Zyxel firewall and pfsense together anyway, what's the point? If you want pfsense then you can build your own box anyway and don't need a Zyxel. And if you use the Zyxel "normally" you don't need any extra licences. It runs perfectly well as a professional firewall and I need it because of the router-terminated Ipsec VPN connections to other private networks in the family.

Hello, I don't use pfsense. I do not pay any license for the use of Zyxel.
Best regards.

In principle, no additional firewall licences are required. However, if you wish, special software licences are available for purchase.
I do not know whether pfSense can be used on the USG Flex 200.