Re: problem with my netgear R6900
Did you reset the router to factory settings after the firmware flash?
New firmware sometimes introduces changes that are not compatible with the old configuration.
So a factory reset is the first thing to try if you have problems with new firmware.
Backup your configuration first. Netgear does not advise restoring old settings into new firmware, but if resetting does not fix your problem, and you have complicated settings, you might want to restore them, especially if you roll back to earlier firmware.
Also make a note of your settings, screen grabs can help here.
My network DM200 -> R7800 -> GS316 -> PL1000 -> Orbi RBR40 -> Orbi RBS50Y -> RBS40V