My Archer was bought on september, 9th, 2019, to replace my old TL-WR849N, that works flawlessly until today.
My service provider uses PPPOE.

The Archer C9 v5.1 worked very well for the first two months.
On november, 15th, 2019, conections problems started and I had to bring the old TL-WR849N back, and it's working very nicely.

Every time I try to put the Archer as my router the conection is VERY bad, but that does not happen with the TL-WR849N.

For my surprise, the problem seems to happen only when I use the Archer on PPPOE mode.

The Archer works perfectly on AP MODE and Dynamic IP MODE, both modes with the WAN port attached to the LAN port of the TL-WR849N.

I've tried to solve this, with no success, these ways:
Factory reset
Factory reset followed by a firmware DOWNGRADE and another factory reset

Factory reset followed by a firmware UPGRADE and another factory reset

Then I stated trying to disable funcions:
NAT boost, igmp, firewall, upnp, wifi, USB share, gigabit wan 

I even tried to put a network switch between the Archer and the provider cable, to use it as a "filter" ¯\_(ツ)_/¯, but had the same problems when activating PPPOE.

I really would thank any new insight. This modem is a USA model and I don't think I'll have support here in Brazil.

Thanks