Download Magisk from github and just flash it like you did your custom rom. You need to redo this after each rom update/reflashing your recovery. Afterwards open the Magisk app and follow the onscreen instructions (install via root)
Although this method of installing Magisk is not recommended anymore it’s the only one that worked for me. Following the regular instructions always ends in a bootloop/non responsive display for me
Try creating a tvshow.nfo only containing the TheMovieDB link in your root House of cards folder
Do the same for each season folder, naming the file season.nfo and paste the corresponding TheMovieDB season links into them