Guest Wifi network stuck on old DNS configuration Orbi RBR750 in AP mode

If you set a static IP address configuration for AP mode, you can set a specific or custom DNS address to use on the RBR while its in AP mode. 

Static IP Address Configuration for AP Mode | NETGEAR Communities

While in AP mode any connected clients get DNS either from the custom DNS set on the RBR explicitly or if dynamically, will come from the host router up stream that is set there or automatic detected from the ISP. I believe any Guest Network side of things would also be automatic from the host router. Not sure if setting static and explicit DNS on the RBR in AP mode changes for the Guest Network side of the RBR.

If anything factory reset the RBR and setup from scratch with a ethernet PC and set up DNS there, then change to AP mode and test.