Sorry, I am not sure. Maybe someone from TP Link can comment.

Nothing I see. You can try connecting only the computer to one LAN port with the other LAN ports empty and re-test. Also be sure to power cycle the modem, router, and PC between tests.

Since you got a new modem, connect a PC direct to the modem via Ethernet cable and see if that works, If not you need to get that working prior to bringing the WA801ND into play. It could be your ISP...

@TheRiddle You may have bad unit. First try changing the channel from auto to one of the available channels and also the channel width. You may be on a channel that has some type of interference in...

If you know the MAC address, can you block the device?

How stable is your electric? Is your router on a UPS? Some electronics are more sensitive to power changes and I have seen strange things when a UPS battery is past its life.