The J1 hardware is old but still capable as a basic ADSL2+ modem or a secondary access point. A firmware update won’t give you gigabit Wi-Fi or MU-MIMO, but it will close security holes and improve reliability.
Index of /pub/ADSL/DSL-2750U/Firmware. This firmware is only compatible with hardware revision С. Please take a look at string "H/ D-Link FTP How do I update my device's firmware manually? | D-Link dlink dsl2750u h w ver j1 firmware updated
If you want, I can:
Select (or Firmware Upgrade ) from the left-hand menu. The J1 hardware is old but still capable