limited wan speed

I'm on a 100Mbps fibre connection (yes, internet in my country is slow). My router tends to work perfectly for a while - 100 up and down but keeps dropping down to 2Mbps after a few hours. I haven't worked out the exact timing. I am using the latest ASUSWRT-Merlin firmware (386.3_2), but the...

I have a Netgear R7000 hooked to a Arris SB6183 modem, the router always reports a connection 100m instead of 1000m. When I connect anything else to the modem it connects at 1000m as it should. Every cable I have tried (cat 5e) connects at 1000m with everything but the router so I don't think I...