sorry I am so late on answering your post.
the weather outside is ugly, so I decided to read old threads/posts to pass the time.
If you have access to a cat. 5 cable tester use it to verify that your cable connector terminations are good. A bad connection, crossed, shorted, loose and incorrectly terminated in the connectors may cause your problem
Category 5 industry standards are 100 meters (325') maximum cable length and should work providing you have met ALL of the installation requiremets such as:
no 90 sharp bends. there is a maximum bend radius allowed.
no kinks.
no stretching the cable.
no staples.
no cable ties.
cable terminated as 568B or 568A.
and etc and etc.
If you exceeded the 100 meter Length the cable Length would cause the router to function as
10base-t vs 100base-t probably just on that cable.
Having said all of the above and as a cable (voice, video, data) installer of many, many years and with a high quality cat. 5 certification tester I was curious about what the test results would be if the cable was kinked, stretched, twisted knotted and the RJ-45 connectors were made with the cable untwisted and not in the connector as required.
I really messed with this exact 100 meter length test cable and the end result was that the cable passed the certification tests no mater how much I abused it.