ISPs like BT, TalkTalk, or Sky (depending on your region) sometimes change their VLAN or PPPoE standards. A firmware update adds new "Profile" options to the WAN settings, allowing your D305 to connect to modern fiber networks.
Before starting, you must obtain the correct firmware file for your hardware version from the Tenda Global Download Center Search for on the support page. Tenda D305 Firmware Update
However, if your router is working fine and the only "update" available is the same version you have, do not risk it. ISPs like BT, TalkTalk, or Sky (depending on