Very important is signal level on your computer, good tester is inSSIDer, you can search in internet this software. Next, what is your Wireless Settings on routera, give us screenshot from this page.

TPLINK appear to be ignoring this problem!. Maybe this problem results from hardware not from firmware.

Before upgrade, was it good setting? Devices worked properly? Czy przed aktualizacją była dobra konfiguracja? Sprzęt działał poprawnie?

So we td-w8968 can only DynDNS at moment I confirm, but if you have in your network some computer, you can install client application from another DDNS to refresh addressing,

This is problem with loopback in this configuration. In local network you have to use local IP address.

Please make default factory configuration (button reset) and then make connection by browser.