In-place sewer cleanout tee replacement?
You could dig down around the area and cut out the broken fitting. If you cut out wide enough you can install a new replacement fitting that has a stub of pipe sticking out each side that just fits down between the cut ends of the existing not movable pipe.
You can then use Fernco type couplers on each side to join the two stub pieces to the existing pipe. Note you will have to dig back far enough on the two ends of the existing pipe to be able to slip the couplers in place before putting the new fitting with stubs in place.

Picture Source
You want to make sure to use the type with stainless steel clamps to resist rusting and that have the metal shielding as shown above to resist the heavy earth load from above.