If its from Optus then you purchased a device that never was owned by the person who sold it.
All Fetch devices from Optus are on rent/lease and can only be activated against the Optus service it was sold with or in this case you can have a mini for $5 a month against an Optus Post Paid service.
You cannot activate a 2nd hand Fetch box that was from Optus.
Its a paperweight if its a 2nd hand Optus device.
Unless you bought a refurbished one from an established company (i.e. JBHiFi) then as mentioned its a paper weight. Sorry, you got scammed.