That's weird...can you update driver of network card on your linux?

Default username and password are both admin. Did you change username or password? If they are rejected, to login http://192.168.1.1, you may have to reset it to factory defaults.

W9980 is working as a modem router or router only? When that computer is not connected to LAN 4, what's the status of local area connection? Cable unplugged? Unidentified network?

TD-W8968 V4? A brand new router? Firmware is up to date? With Wi-Fi connection, normally, what's the main internet activities? Browsing webpages? Streaming video?

Did you contact your ISP to test the line? Go to Advanced Setup---ADSL, change ADSL Mode to ADSL2 or G.DMT. If ADSL light still keeps flashing intead of solid on with telephone line plugged in, may...

Is your MR3220 running the latest firmware? Modem works when it is connected to PC directly?