Have you got the latest firmware downdate? SG4B1000B316 issued a few moths back. Prior to this my HH6 was a source of misery and I was about to buy a third party device.
After that Firmware version downdate split SSID and speeds worked and have been troible free (so far).
Check your new devices firmware version.
I have had a HH6 since inception (part of testing) and never had the problems you describe
When you look at connected devices, does anything show up as "unknown". Does this hub have any powerline technology built into it?