You're trying to connect the RBR50 to an existing router?
Is that what you're trying to accomplish? If not, maybe explain what you're setup is and what your trying to accomplish. Linking to something else doesn't explain what you're trying to do with your setup.
If you are trying to setup orbi with an existing router, you'd want to use access point modem on the rbr. That allows your existing device to run dhcp/routing functions and uses the orbi for wireless/ethernet connections.
This "B Box" appears to be required (cannot be removed). This is in Belgium, correct?
I, personally, would select "Option 1", i.e. put the Orbi into Access Point (AP) mode and let the B Box continue to administer the network.
PPPoE networks are more complicated than DHCP networks (in my opinion, since I have never used one in 20 years), so my guess is that using the Orbi in place of the B Box is not likely to be possible.
Does this network have a user forum where you could post about attaching an Orbi?
I'm indeed from Belgium and i can't remove the Bbox3 device from my ISP configuration setup.
I absolutely don't want to use my RBK50 as Access Point because i want to use VPN features, configure my NASS and honestly regarding the selling price of the device being able to use it only as AP would be a real scam.
Proximus have already set up a set by step process to configure private routeur. I'm really expecting an asnwer from Netgear as this kind of question sound pretty basic to me. Proximus allow to use a private routeur to get a second IP. I don't understand why this wouldn't possible with an RBK50?
Those are good reasons to want Orbi to be in router mode. The ISP will be key to providing detailed instructions. Outbound VPN would work with the Orbi in AP mode. Inbound VPN definitely needs router mode and will never work if "behind" another router. The Orbi will need a public IP address. Good Luck
