Help : RT-AC68U slow download, fast upload

Hi
Just got my first Asus router, however I'm stuck !

All LAN ports have been tested : slow download ~0.5 Mbit/s , OK upload ~20 Mbit/s
Speed test of ISP (without AC68U) : download & upload ~35Mbit/s
Wireless connections : slow/weak - (haven't focus on this since i don't get the LAN to work)

Firmware tested :
- First 376_1663 (firmware upgrade during first setup) & tried reset to factory defaults
- Downgraded to 374_5656 : reset to factory defaults, restart...

Help : RT-AC68U slow download, fast upload