I am working with Optus Fetch for past 2 years and this is a very common issue with a simple solution !! You just need to turn the fetch box off and den turn your modem off !!! Wait for a minute and turn on your modem, as soon as you can access Internet on other devices, turn on your fetch box !! This will resolve your issue !!
I work for the Optus TV & Content Technical Support team and provide advice on Yes Crowd voluntarily.