[cable]

The SkyQ box method of connecting to mini boxes is not simply a WIFI repeater but a mesh wireless system. It works by the main box using its 5GHz WIFI radio to broadcast its own network leaving the 2.4Ghz radio for connection to the user home router. The Sky router would have a secondary hidden 5Ghz SSID that connects to the SkyQ box. its a feature of the hardware and software being able to support multiple SSIDs through Skys customisation of the software. Other providers don't have this SSID broadcasting from their equipment so that why it doesn't work with other providers routers.

Sonos utilises something similar for its speakers.

There is no real benefit for this with SkyQ* which uses the 802.11AC/WIFI5 standard, the new Sky Max hub uses a newer specification of WIFI (WIFI6) and the 5GHz radios get used to deliver the faster speeds (and are not being used solely as backhaul). Typically unless you live in a big house, SKY WIFI MAX is overkill for most people, so don't be a sucker for the marketing. WIFI6 helps regardless due to MU-MIMO and beamforming.

*As with all generations of wireless, maintaining older devices compromises speed. WIFI6 also requires all end devices to support WIFI6 as well to get the full benefit, typically high end devices released in the last 3 years.