You are talking about Diastasis Recti. It is caused by a weakening of the fascia (connective tissue sheet) down the center of the abdomen/tummy. It is fairly common after giving birth. But some research is suggesting that the tissue was already weak or thin prior to the birth. In any case, connective tissue tends to take longer to change than muscle tissue. So, the timeline for drawing the two sides back together is a long one. The success that I have seen with repairing DR has been with consistent effort to maintain good posture and an engaged core. Learning how to engage your abs and maintain that engagement is key.
A starter is just learning how it feels to have an engaged core. I teach a technique where you would exhale as if trying to blow out birthday candles on an endless cake. You should try to force more air out by clenching the abs and bringing the belly button toward the spine. Doing this correctly is a bit harder than it sounds here. And having someone demonstrate and observe you doing it, goes a long way to learning to do it well. You need to be able to engage while exhaling and inhaling once you have figured out what is to engage these muscles (mainly transverse abdominis and ext./int. obliques).
Once you can do those two, it helps to learn to “pull up” the pelvic floor as well. You can look into Kegel exercises to help with this.
Now you attempt to do all three as much as possible throughout the day and when at rest as well. Part of getting the two sides to stay closer is getting them closer as much as possible. Unfortunately, wearing a splint doesn’t appear to be much help (except cosmeticly). I think that may be due to the lack of neuromuscular stimulation, which is important to stimulating the tissues to change. So, you need to make it a habit to engage the muscles above more and more.
I could go on, but you should try to find an instructor in your area who is confident in Post Natal exercise. Your OB/GYN would be a good person to ask for a reference to find such an instructor. If they don’t have an instructor to refer, check out your local rec center. Keep asking for a referral until you find someone. If you have an instructor that you like to work with, encourage them to learn more about helping you (and others) with this. I do teach instructors a few exercise programs that overlap on this information. Have them check my profile here at IdeaFit to get in touch with me if they want some help.
Lastly, genetics and prior core development do play a big roll in how successful a person with Diastasis Recti will be in improving/repairing the gap. Consistent effor is the only other factor and the only factor that you will have any control over at this point.