DIAGNOSIS Asymmetric speed issues point to Quality of Service (QoS) settings, a firmware bug, or ISP throttling, not a local connection problem.
FIX Disable QoS. Log into the router interface. Go to Advanced > Setup > QoS Setup. Set 'Enable WMM (Wi-Fi Multimedia) settings' to unchecked. Set 'Disable QoS' to checked. Click Apply. Re-run your speed test. Also, ensure no bandwidth monitoring or traffic meter features are enabled, as they can cap uploads.
IF THAT DIDN'T WORK Perform a factory reset. Use a paperclip to press and hold the reset button on the back for at least 7 seconds until the ring light flashes amber. Set up the router again minimally, skipping any QoS prompts. Test speeds before re-enabling any advanced features.
HEADS UP Some ISPs, particularly cable providers, have significantly lower upload speeds as part of their service plan. Test with a computer connected directly to your modem to establish your ISP's baseline upload speed before blaming the router.