Thanks for the information. So as you said it is possible that those costs are calculated at the end of the Nebenkosten cycle and I just need to be patient. But Wincasa never hinted toward this being the case. They mostly said they never got the information from UPC.
This page seems to concur somehow https://www.mieterverband.ch/mv/mietrecht-beratung/ratgeber-mietrecht/top-themen/tv-internet.html
I am also not sure it that legal to withdraw the rent (Consignation du loyer) as it seems the condition for this are the apartment itself should have some kind of defect.