okay it's called the cross body or cross leg piriformis side stretch here is a video https://youtu.be/5ohZJCXtIvY?feature=shared (I do not endorse chiropractic 'medicine' but that's a good short video of the stretch)
I had really bad sciatica and could hardly walk at times but doing a few of these would really open my whole back up for a while.
I discovered that stretch doing this low mobility yoga routine: at 8:30 she does a 'forward fold' that i think is the same thing. Nothing else came close to helping me but of course it's sciatica and you might be locked up by a different muscle so ymmv.
good luck mate!
can just freeze eggs until you're ready for them