Clients on different bands cannot see each other

FW version: NTGX24_10.17.03.00

I have started using my M2 as a complete routing solution, replacing my old Asus RT-AC56U. I currently have 2 computers, a bridge router (the RT-AC56U), and tablet connected to the network via 5GHz and 2.4GHz bands, however I am having difficulty with interoperability of the devices when they are mixed between 5 and 2.4G bands.

The M2 dashboard shows all clients in its list, and which band they are connected to. My 5G devices can talk to 5G devices, and 2.4G to 2.4G, however not 2.4-5G or vice versa. The router has default settings, aside from SSID and key changes. I cannot find a setting that should prevent operation of the devices. Weirdly, I can ping across bands, however any application or web request to a device in a different band fails (eg using VLC to view shared folders from a computer to the tablet, or loading the bridge router login page in a browser). These functions work fine if the connection method to the M2 is the same.

I have just tested a wired connection from one of the computers, and this does not overcome the issue (it can still ping, but not serve media or load the bridge router login page). 

I didn't experience this issue with the RT-AC56U, and by all other accounts that I have found, there should be no reason devices should not connect out of the box when connected to the same router. I have factory reset the router with no change.

Would anyone have any advice?