Ceramic floor tile? I would not put a sealer on it unless it is specifically made for floors and will not yellow them. Sealers eventually have to be stripped and re-done. That is a lot of work.
Lots of people swear by cleaning them with vinegar water. Drying them with a buffing motion helps greatly.
Tile floors? This will help: https://www.hunker.com/12574792/natural-ways-to-make-tile-floors-shine
Here are some tips https://www.hunker.com/12574792/natural-ways-to-make-tile-floors-shine
If you're asking about old bathroom/kitchen tiles, the first step is to determine if they're coated with grease and dirt or soap scum. Rub a section with your fingernail to see if there is shine under there. Clean and scrub the tiles, rinse well. This is a huge job! I just found this, which will help immensely
His "test" showed OxyClean 2 cups/gallon hot water sitting for 60 minutes worked best. I am STILL looking for ways to do vertical surfaces lol!
It possible your tiles were damaged by earlier cleaning products. You could put on a coat of car wax, redoing it yearly.
This is extreme!