Rob Morley wrote;

I'd remove the remaining mortar and cut the pipe down flush, clean
everything very throughly, apply a large fillet of silicone sealant
then build up a ring of mortar so it looks like it did in the first
place.

My thoughts exactly. I had a similar leak on a cast iron soil pipe. Just
built a box around it as formwork & filled it with fine concrete. That was
10 years ago and it's still K.

Dave