Workaround for 2.4Ghz connection issue on BT Smart Hub 2
The "Smart Hub 2" comes with a range extender which BT choose to call a "Disc". The "Disc" has an Ethernet socket on the back. The problematic 2.4Ghz device will also (very likely) have either an Ethernet or a USB socket on it. Set up the "Disk" as per BT's instructions. Physically connect the "Disc" to the 2.4Ghz device with an Ethernet cable (via a suitable adaptor, if necessary). The device should now work.
The solution does rely on being able to place the 'Disc" close enough to the 2.4Ghz device to connect them by cable.
All you're doing is taking the device's 2.4Ghz wireless connection out of the equation. The device is now connected to the "Smart Hub 2" via 5Ghz and Ethernet. It worked for my Naim mu-so Speaker, so I expect it will work with other 2.4Ghz devices.