How to enable Gues WiFi SSID without primary SSID and how to enable RADIUS for Guest Network

Netgear R8000P is a powerful router, but offers extremely limited features. There is no way to create port-based and/or tag-based VLAN for connections of choice, except for IPTV-specific VLAN feature, for which I do not care.

I am tryinig to increase security of my WiFi via Guest Isolation, which works relatively well, but Guest Isolation SSID requires the main SSID to be ebabled. Why? Why can I have just on SSID with Guest WiFi Isolations and no others?

Then there is the issue with RADIUS server feature applicable only to main SSID, but not Guest WiFi isolation SSID.

Could someone recommen me a similar router that supports all the features I mention and can be flashed to DD-WRT. I heard the non-P version of R8000 can be flashed to DD-WRT, but does it have weaker WiFi. I only use 2.4Ghz WiFi around my residence and it is always enough.