Guest Wi-Fi using the same Ethernet LAN as an IP video security camera network?

Question

This hotel has a private network for all of its PCs, using the 172.20.7.0/25 subnet.

A second, separate network for IP cameras uses the 192.168.1.0/24 subnet. All the cameras are connected using unmanaged POE switches and outdoor wireless access points working on the 5 GHz band, offering pretty decent bandwidth and supporting high throughput as proper channel selection was used.

There is a Windows server with two NICs recording video from the cameras. The first NIC has an IP address on the PC subnet and the second NIC has an IP address on the video camera network so all PCs have access to the server and can view the cameras, but without direct access to the camera network itself. This server has access to the internet through the first network.

Now we need to implement another network for Guest Wi-Fi Access in locations where the video camera network's 5GHz APs already exist. For the Guest network, 2.4GHz access points would be used.

The question is, how can we implement a third network (with a new IP range that must have a DHCP server and a captive portal with network management for bandwidth and user data) and connect it over the old camera network? Can we use this server with a third NIC and share the internet connection over it? I would try to avoid VLANs as I am not experienced with them at all.

Guest Wi-Fi should have by no means any access to anything but the internet.

Any help would be much appreciated!