When the cable modem is installed you see the tech measure the signals and do a few checks. Since it didn't work but does at the other location it means there is an issue with the cable to that point.

It could be as silly as there is some SPLITTER from there to here but as you are now the cable guy, you get to find it.

Frankly I rarely move such things. I use some bridge unit to get the internet to the other location.