internet access problem using router and homeplugs

My 2 PCs are (actually were, for years) connected to internet through modem-router-homeplug. After replacing the router, 1st PC works fine. But, despite a brand-new set of homeplugs (3 pieces), second PC cannot find connection. Used different ethernet cables without any result. What am I overlooking? What should I look at? The shop is not helpful ("It is just plug-and-play, it should work").