To configure TD-8817 as a modem router, what you need is VPI, VCI, connection type, encapsulation mode, username and password. It is unbelievable that your ISP won't provide those info. Did you try...

Your retailer? Or have a check if there is any info about warranty on TP-LINK website of your region.

It is too old and discontinued...so that it is not listed on the website anymore... Why do you want to upgrade the firmware? If no problem with it, just leave it.

What kinds of UPnP settings? As I know, it can be disabled or enabled and there is a Current UPnP Settings List. Do you mean the list goes blank af ter you reboot the router? If you press Refresh...

Access point has no access control feature as it is not the DHCP server and the network is still "controlled" by the main router.

You didn't get a chance to install the driver, right? Sounds like a hardware compatibility issue.