The Saver Day Pass is only valid on public transport serving inhabited places yet, with a few exemptions (Mt. Rigi, Mt. Stanserhorn, Stoos, Klewenalp), not on mountain railways or cable ways.
What concerns your question, the SDP is valid as far as Wengen or Grindelwald, from there you need a ticket. See the SDP validity map here: https://www.mystsnet.com/areaofvaliditymap2023