It will work for a short period of time. After that, the return side of the modem signal level will go up and then start cutting out. Diplexers is mainly used one way only. Not 2 way. Plus having to many splits in the line will cause the same issues. That is when the customer needs to choose what is more important, satelite or internet. I have removed many diplexers because, they have been the main cause of the problems.
This is CABL.com posting #259262. Tiny Link: cabl.co/mbfBO