Archer VR900 V2 drops speed
I have an Archer VR900 V2 router and I am using it in Wireless Router Mode with WAN connected to port 1 from a modem.
My internet connection is 500Mbit/s (down/up) fiber optic.
The problem is, after 2 days the VR900 drops the down/up speed to 260Mbit/s. Rebooting or leaving it unpluged for couple of minutes does not help. Only thing that helps is Factory Default Restore. But, after about 2 days the same thing happens.
When this happens, I unplug the routers LAN cable from modem and plug my computer in the same port and would get 496Mbit/s down/up. So, the modem is not at fault here.
I have changed the thermal pads of the router, thinking, maybe it was overheating, but it did not change anything.
What else could I do to fix this problem?
Edit: the thermal pads I used - https://www.arctic.ac/en/Thermal-Pad-10-x-10-x-3.5-mm-thick-10-pcs/T-Ti39-010-010-35-04
Before assembly, I checked and they are making a good contact with heat spreader.
Edit 2: The VR900 model selected above is not the correct model. Visualy it is the same, but I do not have Internet Telephony.
This is the correct router - VR900 V2