Engine will not start after battery and terminal replacement

My standard 2011 Kia Soul’s positive battery terminal became very corroded and after having to jump it in order to get it to start, I took it to Walmart for a new battery. After they replaced the battery, the car engine wouldn’t start at all. They tested the battery a few times and it was charged and fine, should have worked. They then replaced the terminal (I’m not great with cars or terminology, but I mean the metal connector between battery and cable). They tried again, engine still wouldn’t start. At this point, from what I understand, they “trimmed” the slightly corroded part of the cable leading to the positive battery terminal, but still wouldn’t start. Eventually one of the mechanic used a tool to “ground” the battery and it started. They tightened all the terminals and sent me on my way, 4 hours later. It turned on and off 4 times that day, then sat for 24 hours. Now it won’t turn on again. The lights, dashboard, radio, everything else turns on, but when the key is turned, the engine doesn’t do anything at all. What could be the issue and how much money am I looking at spending? My warranty won’t cover “maintenance”, so they likely won’t cover this.