Guest Network Not Visible to Guests

The latest Orbi 850 firmware doesn't have the option to hide the SSID broadcast anymore: https://community.netgear.com/t5/Orbi-AX/No-Enable-SSID-broadcast-option-in-RBR850-firmware/td-p/1844505

I think your guest network visibility problem may have to do with security or other wireless settings.  What Orbi defaults have you changed besides the wireless security password?

Use a web browser to visit http://orbilogin.com (you'll need the admin password set during setup).  This takes you to the Orbi's admin web interface which offers all the control settings vs. the app.

In Advanced tab / Setup / Guest Network, check your Security Options setting.  I use "WPA-PSK [TKIP] + WPA2-PSK [AES]" for guests but "WPA2-PSK [AES]" should also work well with most devices.  Check what this value is set to vs. your primary network.

These other tests are unlikely since your guests can see the primary network, but just in case:

• In Advanced tab / Advanced / Wireless Settings, try checking "Enable 20/40 MHz Coexistence".  I find that changing CTS/RTS Threshold to 2347 maximizes compatibility with older wireless devices.
• The final test is to try disabling AX mode in Advanced tab / Setup / Wireless Setup.  You can uncheck it only for 2.4Ghz (that's what I do).  It would be unfortunate to have to disable AX mode for 5Ghz in your fancy new AX router.