For security and compatibility reasons, you should only download firmware from official sources. Note that firmware is often region-specific (e.g., EU, AU, or MEA) and must match your router's hardware version (e.g., Rev A1, A2, or B1).
The D-Link UK/Europe Product Page provides manuals and firmware specific to the Wireless AC750 Dual-Band Multi-WAN model. dwr-118 firmware download
Only perform a firmware update via a wired Ethernet connection . Updating over Wi-Fi can lead to a "bricked" (unusable) router if the connection drops during the process. DWR-118 Wireless AC750 Dual-Band Multi-WAN Router - DLink For security and compatibility reasons, you should only