There is a tab on the asus router called tools that will show the speeds of the lan and wan ports. At least I think there is I load merlin firmware on my asus routers
I do not think there is anyway to change the speed settings so it should auto negotiate. If this show 100mbps negotiated rate then it has to be a cable there is not much else it can be since you can't change an settings. You would hope it is not a defective port in the modem or router.
If this speed show 1000mbps then I would suspect the ISP.
You should be able to plug your pc directly into the modem to test.