Since the modem can be identified but no internet, did you try to manually configure APN, dial number etc.?

It keeps running for 21 days without power-off ??? It needs a break...

Upgrade latest firmware of WA854RE first. http://www.tp-link.com/en/support/download/?model=TL-WA854RE&version=V1#tbl_j Reconfigure it using method 2. You may use a different web browser. After you...

What are you trying to control? If you are not sure, you can put in 1-65535 as the example here. http://www.tp-link.com/en/article/?faqid=557

Have you tried different cables and different PCs to connect LAN ports on W8980? If none of them on or flashing when you try that and none of them respond when your reset it, it should be a faulty...

You can just select to Enable Firewall or not as I read the user guide of W8968 V3...page 25... http://www.tp-link.com/resources/document/TD-W8968_V3_User_Guide_1910010970.pdf