Tony Seaford wrote How often does the speed drop? It is important to test if the speed is stable or not when your PC is connected to the modem directly and work for a period of time. When the PC is...

I have the same problem. (archer c7 v2, newest firmware here, ipv4 & ipv6 dual stack from the provider) ISP speed is 400 mbit/s. After restarting the router i get the full speed over nat ~ 400 mbit/s...