Router keeps giving wrong IP to a single computer

So, long story short, I found the problem. Not router or computer related. Just apparently forgot my old cable modem was connected to the network still. I have no idea how it hasn't been an issue before. No idea why it is even still plugged up as I have been on fiber for nearly a year now. Guess with getting the new router and something changing on the network it finally came around to wanting to assign an IP again. The only way I isolated it was going back to scratch and connecting 1 device at a time to the router and watching it assign an IP. As I am hooking the wires back in, also confirming where they went. I had one that was blinking but not registering an IP. Moved some dust covered items and there it is. My old cable modem. Well, the least I can say is I have one computer that is back to square one and now I know for sure what is connected to my network, again.