How to finalise on my DVD recorder?

Question

Hello,
I have a Telefunken TDR218 DVD recorder. I have recorded many DVD's
but they won't play on any other DVD player or my computer. I learnt
recently that I need to finalise them and that there should be a
finalise button somewhere, but from what I can see there isn't such an
option on my DVD recorder.

How do I finalise these DVD's on my DVD recorder? Or can I do it on my computer somehow?

Thanks.