You cannot install DSL-2750U V2 firmware on a V1 or V3 router. Doing so will "brick" the device (turn it into a useless paperweight).

: While it initially addressed bugs of its time, later security research (such as CVE-related disclosures in 2016 and 2021) highlighted vulnerabilities in older DSL-2750U firmwares, leading D-Link to eventually release newer patches (like vME1.16/vME1.31) to disable insecure TFTP configuration access. Finding the Official Download