This problem is normally caused by poorly written IoT apps. Whether that is the case with your app, I have no idea. I have never seen a case where the router is at fault. What you need to do is to separate the routers' two WiFi bands to use different network names. This only has to be done while you are connecting these IoT devices. Once done, the router can go back to the same WiFi name for both bands.
What you need to do on the Hub3, once you have logged into it, is to click the cog icon in the Wireless Connected Devices section, against the 5GHz band. In there, append for example these two characters to the end of the SSID name:-
5g
to the network name shown in the 5GHz Band's SSID field. If this was previously (as an example) set to MySSID for both bands, set the 5GHz one to MySSID5g. Click Apply & log out of the router.
Now connect your device that is hosting the app to the Hub's 2.4GHz band (in this example, that would be the MySSID one). Now run through the procedure to add these bulbs. Once they are fully operational, rename the Hub's 5GHz band back to MySSID for this example.
