RBR750 Guest Network visible while guest network is disabled

You'll need to contact NG support or a forum moderator and see if they can get you the FW that one user mentioned, If thats what they really got. I notice they haven't posted since they first mentioned that so not sure how credible that is since we don't see anything else. 

https://community.netgear.com/t5/Orbi-WiFi-6-AX-and-WiFi-6E-AXE/RBK750-Beta-Firmware-4-6-6-11/td-p/2176920/jump-to/first-unread-message

Something else to try, do a full manual re-load of the FW file on the RBS and RBR. After the units are back to ready, use the ERASE function on the RBR and Press the reset button on the RBS for 15 seconds then release. When you see the front LED on the RBS start to blink WHITE, turn it OFF. Walk thru the setup wizard on the RBR and set it up fully. Once it's setup, take the RBS near or to same room as the RBR and  turn ON the RBS and when the front LED slowly blinks WHITE, press the sync button on the RBS and on the RBR and let the RBS re-sync fully. I'd give a full 5 minutes. The front LED on the RBS should turn BLUE then turn OFF. Check the RBRs web page for RBS status and also log into the RBS web page as well. Once the front LED is OFF, turn the RBS off and place in remote location.