I have this same issue and unfortunately I don't think there is a good work around for it. For whatever reason the VPN settings do not currently allow you to specify a DNS server. I even tried to create a VLAN for the ip range associated with the wiregaurd VPN ip subnet but it broke things...
I have not tried out the stock firmware, only the most recent Merlin. The Default IoT Network does not have any options to adjust the DHCP range, unless the options for configuration are somewhere other than the Guest Network Pro tab. Under the guest network tab the only options for the default...
New Asus GT-BE98 Pro, Firmware:3006.102.2. Fresh install and setup, no import of settings. After configuration there was a default wifi network named "Default IoT Network" that I did not have to configure, and has very few configuration options. Only SSID and password. I used this wifi network...