The 081117 should be the version number. But this product is pretty old and it is not on the official website now.

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.