There’s no easy fix. I’d remove the existing tile flooring and check the size of the supporting joists, then 1) check thickness of the subfloor and underlayment, if any...2) then install cement board and 3) install new tile as per the Ceramic Tile Council of North America (CTNA) guidelines.
First, the supporting floor joists must be strong enough to support a tile floor. They cannot deflect or the tile will crack, etc. verify that the joists have a maximum of 1/360 th deflection...there are tables to calculate the deflection based on load, spacing and span.
If the structure is adequate, then 1) check the thickness of the subfloor and underlayment. If your joists are 16” on center, you’ll need a minimum of 5/8” plywood (or equivalent) and a minimum of 1/2” underlayment.
2) Cement board is recommended to be set in 1/4” thinset. (Get a trowel with 1/4” grooves.) Fasten the cement board to the joists using the recommended size and spacing of screws.
3) Install the tiles on the cement board following their precise instructions, including curing times.
As you can see, this not an easy task. Make sure you follow the TCNA guidelines exactly, including making sure the floor joists are strong enough and you have the proper thickness of subfloor and underlayment.
Typically, cracked tile is due to movement (deflection) in the structure.