Orbi compatibility with TalkTalk modem/router Fast 5364

I am considering purchasing the Orbi RBK53 to set up a mesh in my home in order to improve WiFi coverage.  My ISP is TalkTalk and I use their Sagecom Fast 5364-3 modem/router.

My question is whether the RBK53 is fully compatible with the TalkTalk 5364 router?  Netgear suggests that it will work with any generic router and that it just needs plugging into the ISP router’s internet port and everything will work.  However more digging suggests that (i) the ISP router has to be put into bridge or modem mode and/or the RBK53 has to be put into AP mode; and (ii) the Sagecom 5364 cannot be switched to bridge or modem mode.

Does this mean that the TalkTalk/Orbi combination will only work with the Orbi in AP mode?

I have also read that the Orbi App cannot be used to manage the system if it’s in AP mode.  Is this correct – in which case is the App any use at all with the Orbi in AP mode?

Any help appreciated.