No network in one of 8 rooms, although physical connection seems solid
Solution 1:
If a cable has been kinked, the tester may not give you the correct answer.
String a length of cable through the house from panel to room. Does this work? Probably. That tells you that you must re-run the cable to the non working room. This will help you troubleshoot.
Also check rack connections as well.
See if this works for you.