The service light is not on

[KK11]

No internet connection, the "Service" light is off, the rest of the three lights on.

[Chrisee]

@KK11 this page explains the lights on Sky hubs mean https://www.sky.com/help/articles/hub-lights-explained-start 

If you are talking about the wall mounted ONT it is normal to have green lights for power, PON and LAN with the LOS light off. The LOS light indicates a loss of Signal and is bad news.

[JimM1]

@KK11 Few differnet models of ONT out there and either suppliers OR Openreach or CF CityFibre, as to wither the service light is an ON/OFF Lit Green/RED then who knows, do you know what the supplier off your fibre is? Some off the ones below that sky has linked!

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