Prolotherapy/fusion. Sacroiliac joint (SIJ) instability causes 13-30% of chronic low back pain & is a joint hypermobility disorder; due to damaged & weakened ligaments. SIJ belts, yoga, cannabis & massage might provide intermittent relief. Prolotherapy is best natural non-toxic long-term solution to reestablishing pelvic girdle stability. Failing Prolotherapy, SIJ fusion provides a low risk definitive solution. .
Prolotherapy. I have had much success treating sacroiliac (si) & other joint instabilities with prolotherapy - aka 'stimulated ligament reconstruction': injecting si ligaments with a mild irritant (could even be a form of sugar & water). The body 'thinks' it has a new injury and rushes to 'repair' the ligament - laying down new fibers, shortening & strengthening the ligaments. Osteopaths & mds can both do this.
Fix? The only fix is a fusion. Be careful as I have seen many unhappy patients after such an operation. Are you sure the joint is unstable and not just arthritic?