What setting on a thermomix would create a simmer?
(Disclaimer, I don't have thermomix)

Simmering is usually just below boiling temperature (100c), so I would set at 80c and see how it goes depending on the marmalade consistency.

Most recipe I've seen just say to simmer the marmelade; no set temperature.

I'd also look at the Thermomix user manual or user community to see if there are instructions or marmelade recipes