And this only happens when you plug in the ISP cable?
Ya, something on this router is messed up. This should run in any mode and any WAN port speed. I just had my 4500 online a couple of weeks ago and I set all my routers to 1000Mb and use Single mode N 5Ghz.
Do you set 1000Mb on the 826L?
I wonder if you should take the 4500 to a different location with a different ISP modem and test.