Just a quick heads-up in case anyone else is having trouble connecting with the above router (UK)
Just upgraded from a Homehub 5 to 6 and couldn't add any devices. Same SSID, settings etc.
Turns out there are different Modes you can select on the Homehub 6
Mode 1: Gives you the fullest featured wireless, maximising speed and range
Compatible with g/n and a/n/ac
Mode 2: Some devices can sometimes struggle to use the latest wireless features. If you have any devices that can’t get a good wireless connection, try using this mode. Selecting this mode may reduce the wireless range and throughput but improve connectivity.
Compatible with b/n/g and a/n
Mode 3: Change to this mode if your wireless devices don’t perform well using mode 1 or 2. Selecting this mode will reduce the latest features as well as wireless range and throughput compared to Mode 1 and 2. It'll enable the broadest range of devices to operate on the wireless. The features that are reduced include: power saving procedures, traffic aggregation methods and Multiple Concurrent data streams
Compatible with b/g and a
Switching to Mode 2 allowed me to connect all my devices. (Looks like Sonoff need type b)
It worked for me so just something to try if you're having problems.