Credit: https://github.com/Anime4000/RTL960x/discussions/250#discussion-6288339

• OpenWrt router with:
  • SFP port(s) (any speed) with I2C bus
  • Packages ethtool-full i2c-tools
• SFPTotal Wizard (Windows software, only used to calculate checksum at 0x3f)

1. On OpenWrt
   1. Dump EEPROM:
      ethtool -m [ethX] raw on > /tmp/sfp.bin
2. On Windows desktop
   1. Copy EEPROM to Windows desktop (run in Windows Terminal):
      scp -O root@192.168.1.1:/tmp/sfp.bin .
   2. Open sfp.bin in SFPTotal Wizard
   3. Set the bits in SFPTotal Wizard according to the EEPROM Bits table below
   4. Take note of the checksum in 0x3F (first green byte)
3. On OpenWrt
   1. Set the SFP EEPROM bits in OpenWrt according to the EEPROM Bits table below (you will need to adjust -y 5 to match the i2c bus on your system)
      • i2cset -y 5 0x50 0x06 0x00
      • i2cset -y 5 0x50 0x0a 0x05
      • i2cset -y 5 0x50 0x0c 0x1f
      • i2cset -y 5 0x50 0x3f 0x[Refer to SFPTotal Wizard]
   2. Physically re-seat the SFP
   3. Verify the new speeds are detected:
      ethtool -m [ethX]
      

              Supported link modes:   2500baseX/Full
                                      1000baseX/Full