Yes, this is actually a tri-band system. It has three separate radio bands: a 2.4 GHz band with 574 Mbps and two 5 GHz bands with 1,201 Mbps and 2,402 Mbps transmission rates. The third band is therefore a second 5 GHz band that is used for additional capacity and better mesh backhaul connections.
The following security type can be selected in the WLAN settings under Advanced WLAN settings: "WPA2/WPA3 Personal, mixed". It should therefore be possible as long as all nodes in the mesh support this security type. The Velop WiFi 6 AX can therefore do this.
The Velop can do everything, these options are nothing that the user has to set, that's why you won't find them. What you do have to look for is that the ISP router is set to passthrough. Not all of them can do this, for example the Sunrise box or their Fritzbox are restricted in this respect and you cannot operate your own router with Sunrise, which I am still angry about today.