You can't use an extension cable, it won't work.
You can buy 8m video cable.
Also you may try to run supplied video cable along the ceiling side.
What vehicle do you have?
The straight plug is the only one that will fit through tight grommet holes/boot covers to hatchbacks etc. The 90 degree cable can't be pulled through tight spaces.
For the updated SG9663DCPRO+ when it comes to the longer 6 meter cable make sure the straight plug goes into the rear camera and 90 degree plug goes into the front main dashcam.