My previous router was TL-WR840N. It gave the exact speed my ISP provided, but then I bought the Archer C64 router and connected it to the modem. Then I checked using the wifi and also the ethernet cable the speed drops to 1mbps or less everytime but when I use the old router of mine then the speed gets up instantly. What do I do? I've also replaced the Archer C64 with a new one but still the new Archer 64 shows the same problem, speed drops to 1mbps or less. I tried both 2.4 and 5 gHz channels but it's always the same.

please help me.