What sticks sausage stuffing together?

In sausage, meat is "stuck together" by a network of cross-linked myosin, a rather "sticky" protein present in meat.1 This network of proteins can be developed by kneading, much like the gluten in bread. However, myosin only develops properly given enough salt. Thus I see two mistakes you could have made:

1. Insufficient salt. You need about 2% of the weight of the sausage in salt. Your recipe measured salt by volume. This can be problematic as different salt products are more or less dense. If you have a scale, try using weight the next time around.

2. Insufficient kneading. You say in your comment that you mixed the meat by hand. However, you need a lot more than mixing. Give it a good knead, either in a mixer or food processor, or by hand, until it becomes tacky, sticky, and slightly more homogeneous.

In addition to these factors, I am a bit concerned by your recipe. Breadcrumbs, apples, and cider won't really help make sausage: these fillers will only disrupt your myosin network. Try a more basic recipe of meat, salt, and any herbs and spices you like. After you master that, you can try to introduce larger mix-ins like apple pieces. But whatever recipe you follow, remember salt and kneading. Without them, your "sausage" will be nothing more than seasoned mince.