I'm putting fair money on it being a PC / cabling issue to the router.
If you are getting over 200Mbit after swapping to the gig plan using the Ookla Speedtest Windows 10 App from the Windows App Store (not in your browser) then you will have been correctly provisioned on the Spark side.
And after working with @Talkiet at Spark all the issues we ever had bar one was end customer related.
My issue with trying to diagnose these sorts of issues is you always spend a really long time proving it's your hardware and not the Spark connection (this also applies to most other major providers).