Try running N mode only, and see what happens. I'm not very familiar with your router, and what "Mixed Mode" actually implies. When you say everything is ok, have you done a speed test? What kind of bandwidth are you seeing. You can download a free app called speedtest which will allow you to test your actual bandwidth over WIFI and 3G. Run the speedtest with WIFI turned off, then turn WIFI on, and run it again. Please post your results. With n only g only and mixed mode.
My results have been on average
3G 1.2MB down, and 900k up
802.11g 8MB down 2-4MB up.
802.11n anywhere between 0MB down (test failed) to 1.2MB up but connection goes up and down about 5 times during test, and 0 up (test failed) to about 400k up. Its very erratic, sometimes the test wont even start. On the occasion where the test actually runs to completion, the needle indicator is very jerky, indicating that the conncetion is dropping, whereas when running tests in G mode, the needle is very smooth in both directions.