First of all: check the above. I agree with Brianheck there may be transmission on the pc(s) you don't see.
Try with one PC leaving the other off. If no luck, try the other.
If both PC's fail, and there is no good traffic, there is a problem woth your line.
Still no fix? The next stap is to reset your router to the factory defaults and reconfigure it. This way you are certain it has been configures correct.
If all fails:
The router displays your profile I Guess you have a 5600K/512K profile.
The router reads your profile, it does not measure it.
It can happen you got noise on the line. This makes the fact every dataframe has to be sent multiple times before it got transmitted.
Lowering the linespeed to eg. 3Mbit may solve this.
I 've seen this many times before.
In no case you could solve this issue by yourself. It should be done by the ISP.
One thing you can do: be sure the in house cabling is ok.