Your router has a 2x2 MIMO antenna. It can operate at 5Ghz and 2.4Ghz simultaneously covering N and G bands. So something is wrong here as you mentioned.
Your Surface is forced to use the G band instead of N. This usually is achieved by a handshake between the devices. This is what I suggest.
- Turn off all your wireless devices,
- Reboot router.
- Logon to your router and make sure nothing is connected wirelessly. This ensures antennas are free and not locked into any bands.
- Power on surface and check your connection speed/band to see if it uses faster. N band.
if still shows as slow 50 mbps, that could mean the auto negotiate for band selection is not working right.On surface you can force N band selection. this is done through adapter configuration and settings. if you cannot find this I can send you details later when I get home.