How to block access to Adult content on RBK22 Orbi Router?

As far as I know, the base Orbi has no mechanism to even define, much less block "adult content".  There are ways to block specific web sites or web sites with specific words or letters in them.  On the web interface, this is on the Advanced Tab->Security->Block Sites.  My guess is that this will prove inadequate for your needs.

Netgear offers a subscription service called "Disney Circle" that is designed to do some of what you seek.  I have not subscribed myself.  I believe there may be a free trial period.

There is an entire community forum dedicated to Disney Circle which may provide information:

https://community.netgear.com/t5/Circle-Smart-Parental-Controls/bd-p/en-home-circle