I suspect it's contrary to your objective of having attractive tile on which to walk, but there are a number of grip-enhancing paints on the market. Many are described as driveway paint with grip or driveway paint with grit. 

Other links suggest adding job-specific grit to paint and from there one could extrapolate that the same grit could be added to a clear surface treatment.

Grit embedded tape is also on the market, far less attractive than a painted surface (opinion) and subject to unsightly wear.

An additional thought popped into my alleged mind. There is a product called [Never-Wet][1], which is a hydrophobic coating. It's marketed under the Rustoleum™ brand and available at many big-box stores and [Amazon][2]. I have a package (somewhere) but have never used it. I recall the package instructions indicating that there's a slight cloudy effect to the application on some surfaces, primarily fabrics. I suspect that would not be a problem on the tile.

As water is creating the slippery effect, perhaps Never-Wet would remove the water and therefore the slippery.

A final resort would be to hire a portable sandblaster vendor or device and remove the slippery glaze of the tile. The coloration would likely remain but become subdued. Perhaps even a floor sander or similar device would enable suitable roughness.

[![Never-wet][3]][3]

Photo from Amazon link.


  [1]: https://www.neverwet.com/
  [2]: https://www.amazon.com/Rust-Oleum-275185-Industrial-Repelling-Treatment/dp/B00FTOEQUC/ref=sr_1_3?keywords=never-wet&qid=1652492155&sr=8-3
  [3]: https://i.sstatic.net/yY0ow.jpg