If you are on version 1.04 or older, upgrade to 1.06. Avoid 1.08 unless you need a specific security fix mentioned in the advisory.
Check the sticker on the bottom or back of the router for "H/W Ver: C1". firmware d-link dsl-2750u h w c1
Updates typically include patches for the built-in SPI firewall and protections against DoS attacks. 3. Step-by-Step Firmware Update Guide If you are on version 1