RBR750 and ESP32 based embedded devices

If I read the Espressif documentation correctly, these ESP32 devices have a combination Bluetooth/WiFi capability that is entirely 2.4G.  Thus, it cannot detect or be influenced by 5G signals.

Disabling ax on the 2.4G band seems reasonable.  Although it is supposed to be backward compatible, there have been posts describing how products suddenly worked with ax was disabled.

IP subnet and DHCP options likely have nothing to do with the underlying problem if the device is not actually connecting to WiFi.  Perhaps attempting to connect to the guest WiFi with no password might be something to try.

There is a definite sequence of steps that devices go through when connecting to WiFi:

https://netbeez.net/blog/how-wifi-connection-works/  

Using Wireshark and a WiFi adapter that can be put into monitor mode will allow examination of the actual progress, from the initial Probe Request through to Association.  The DHCP process begins after Association.