What's the firmware version of your TD-W9980? Could you show us a screenshot of the Firewall settings page? When you mentioned the devices connected to LAN cannot be accessed, how do you access them?...

I think you can send an email to tplink tech support( Support.eg@tp-link.com) to let them check it out.

This sounds like a hardware issue. You may try replace it if it is still under warranty.

The subnet mask 255.255.255.0 would be fine for home network.

You may do a hard reset by pressing the reset button for 10 seconds. If problem persists, you may contact tplink tech support to get help.

Did you also set up the username and password for the customer? If you can do that, I think the device would be plug and play when it is connected to the cx's place. The VPI, VCI and VLAN is actually...