New orchid.
 

Does a dendrobium trantuania require a winter rest and if how long?

The recommendation from one of the best growers I know is for water and dilute fertilizer year round - so no rest - with cool-intermediate to warm temps and Cattleya-like care. If your conditions are cooler and darker in winter I would cut back some on water and fertilizer, but nothing like a true winter rest.

I agree with what PaphMadMan told you. I do not believe this orchid needs a true dry winter rest.