I had an Archer C1200 and installed it as a router.  I have around 20 devices behind NAT, and actually I just wanted it as a router so I disabled AP, enabled LAN DHCP server and connected PPPoE as internet.

At the start the download speed is good.  However after several minutes it became difficult to get IP and connect to its web interface.  It took a long time to get the login page.

Then I rebooted it and tried to ping it.  Here is the ping data:  https://pastebin.com/ipC4NbyJ

You can see that around 5 minutes the ping value became high, and the download speed became very slow (from 300Mb/s to 3Mb/s).

Not sure if it is settings problem or what.  When acting as an AP it was good and stable.  However as a router it seems so weak...