Fitting Topravit Loren 2 WC

This WC has to have the pan and cistern assembled before being placed pretty much blind over the soil pipe opening in the floor.
The floor is self adhesive vinyl planks stuck on hardboard which is nailed to boards underneath.
Is it best to bed in plumbers mait or silicone and then screw down?
Connector 80634
is a good fit into the soil pipe in the floor. Do I just push it in dry?
TIA for any help.

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