Low back pain is complicated. I'm not a physical therapist so I defer to more knowledgeable practitioners when it comes to mobility.
Here are some quick fixes I've found to help with low back pain, as well as maintaining mobility to stay in good positions during training.
1. Hip External Rotation 1-2m/side
Starts at 2:40
Emphasis: keep spine neutral, don't just fold over/round onto the ground. This also works if you leg is elevated, but don't get excited and try too high of an elevation.
2. Anterior Hip Stretch 1-2m/side
Starts at 3:07
Emphasis: Squeeze your back glute, keep low back neutral, do not overextend at the lumbar spine (may be the cause of your low back pain so this can help positioning/stability and mobility.)
3. Piriformis MFS 1-2m/side
Emphasis: Relax glute as much as possible, when foot is over opposite knee rock back and forth to target different tissue.
4. Breathing 101
I know breathing isn't a stretch, but it's actually the most important factor to prevent low back pain. Learning to properly brace and NOT overextend during training and everyday life is crucial.
I personally will perform 5-10 deep breaths with my feet against a wall, learn to control my brace and breathing before every training session. Shown at 13:05
Using these three movements, along with proper breathing I've managed to recover from and most importantly prevent low back pain.