Guest Network to Private

Alas, what you want cannot be done.   The first generation of Netgear Orbi (WiFi5) provided an option for devices on the Guest WiFi network to communicate with devices on the primary network:

When the AX (WiFi6) product line was introduced, that option was removed, and has never been provided since.

Originally, one could imagine two reasons to have a Guest WiFi.

• So that "guests" could be told a WiFi password which could be changed at will. (maybe when they leave)  With it being different from the primary WiFi password, changing the Guest WiFi password would not affect any permanent devices on the primary network.
• To set up a network that is separate from the primary network, so that devices on the Guest network cannot "snoop" on the primary network.

The WiFi5 firmware allowed the user to decide what was important (and what was not).

If you want people to access that AVR, you have to tell them the primary network WiFi password.

One might imagine that NAT Hairpinning (also called NAT Loopback) would provide a workaround.

https://en.wikipedia.org/wiki/Network_address_translation#NAT_hairpinning

All Netgear routers support NAT Hairpinning.  When a device on the primary network attempts to connect to the public IP of the router and the appropriate port is forwarded to a specific local IP address, the connection will "loop back" at the router.  It might be worth a few minutes to confirm:

• That a device on the primary network can use the public IP of the router (and the appropriate port number) to connect to the AVR, and
• Then attempt the same thing with a device on the Guest WiFi network.