Welcome to the Bunnings Workshop community @AnaB. It's wonderful to have you join us, and many thanks for your question about sealing porcelain tiles.
Are these tiles glazed? Even glazed tiles are susceptible to tannin staining, so applying a sealer over a glaze might prove to be ineffective. If the tiles are un-glazed, then applying a sealer should help prevent staining. Do a test to see: Apply some water droplets to a tile in the shade. Wait 30 minutes and then see if the water has been absorbed into the tile or if it remains on top. Absorption indicates the tile is not glazed and is porous. If the tiles are porous, I'd recommend doing a test section with Crommelins Diamond Coat Slate and stone sealer. After the product has dried, if there is no discolouration and the sealer is well-adhered, I'd suggest coating the entire area. Make sure to follow the directions on the product for application.