Orbi won't connect with some devices, how to solve? Settings? Attach previous N900 router?

Unfortunately matching your old SSID doesn't mean that everything will "just work" because as you noted, IP addresses will change. Devices usually use DHCP to request an IP address so they need to go through that process again.  For many devices this is automatic and will "just work" but for others you have to manually repeat the setup process as if you're connecting to a new wireless router and if you really did change the SSID.  I don't think there's a reason why a device would be incompatible with Orbi anymore than they would be incompatible with any other particular router.