It you and your tenant each had their own service, then you each had separate ONT's and coax runs somewhere along the line. While your concept is correct ... connecting the first floor outlets to the same cable plant as your second/third floor cable plant should work just fine -- somewhere along the line, I suspect what you think is the cable which goes to the first floor outlet in question is not the actual cable.
A little cable sleuthing is going to be needed here to trace the cable from the outlet to where it actually connects. Have you looked to see if there is another ONT somewhere? There could also be a lo-pass in-line filter somewhere which would have been installed if you both had internet service which would need to be removed.