Upload speed is a 1/4 of what is advertised.

I have the "Connect More" plan with an advertised 300 down and 100 up. My router has a built in speed-test functionality that tests the speed at the modem (not at a wifi connected device) eliminating speed loss due to wifi connection issues. When I test my speed with that function I'm seeing actual speeds of 350 down (yay!) but only 24 up.  That's less than a quarter of what I'm paying for. The fine print said that typical upload speed on this plan was 117 up. 

Any suggestions for how to unlock the full upload speed?