Security light

Okay, see if someone can help me out. I see myself as a pretty good mechanic but I'm at my wits end.
So let me tell the whole story from the beginning, somewhere in June 2019 the security came on and car wouldn't start so I tried the key reset over and over and the battery finally died, so I decided I'm going to get a new battery, so I get a new battery and put it in and it started with no security light and ran for months, well back in late September 2019 the security light came back on, so I have the battery tested and it tested good so I didn't know what the problem was, I checked fuses and left the cables off hoping to reset but no avail, so I decided to buy another battery because computerized stuff is not my strong suit, so I buy another battery and security turns of immediately and works good for a few weeks, and it comes back on and car wouldn't start it was sitting at a gas station, after a few days it started back up and I drove it home with the security light on, it no longer says Buick it now says Delphi on the screen. It'll still crank and start now but none of the gauges work besides the speedometer. Can someone shed some light on this for me?