Auto negotiation on Ethernet is usually quite reliable and should be trusted.
So if both ends, the S905 box and the switch/router, support 1Gbit/s (and both really have to) it's likely bad quality Ethernet cabling. So try a different cable first.
Disabling auto negotiation and forcing a higher speed is not advised.
If you really want to you have to do it on both ends, that means on the switch/router as well. If you can only do it on one end, forget about it.
On how to do it, ethtool can do it, use google to search for it's documentation. As for your switch/router, google will help too, if it's possible at all.
But be prepared for it to fail, at best you'll likely have an unstable connection with a lot of packet loss, most likely the connection will fail completely.