You should already be receiving it if you are Plat. I have never been charged for the premium tier internet, and if they ever tried, I would outline the benefit listed on the Marriott site:

Complimentary Premium Tiered Internet Access
Stream faster with premium tiered Internet access where available at participating brands worldwide including: The Ritz-Carlton®, JW Marriott®, Autograph Collection® Hotels, Renaissance® Hotels, Marriott Hotels®, Gaylord Hotels®, AC Hotels by Marriott®, Courtyard®, Residence Inn®, SpringHill Suites®, Fairfield Inn & Suites® and TownePlace Suites®. Not applicable in meeting rooms.