Open a web browser and enter http://192.168.1.1 or http://dlinkrouter.local in the address bar.
Look at the sticker on the bottom of your physical DSL-3788 router. It will list a Hardware Version (e.g., H/W Ver: A1 or B1). You must download the firmware that matches your specific hardware version. Installing firmware meant for "Version B" onto a "Version A" router will break the device. Step-by-Step: How to Update DSL-3788 Firmware
Many users never update their router firmware, assuming “if it works, don’t fix it.” This is a dangerous misconception. Here is why regular updates are critical for the DSL-3788:
October 26, 2023 Subject: DSL Gateway Firmware Analysis Target Device: ASUS DSL-AC68U (Broadcom BCM63138 Architecture)
Check the displayed in the top-right corner to ensure it matches the version you just installed.