DIAGNOSIS You're probably connected to the slower 2.4GHz band, or a device is hogging bandwidth with a heavy upload/download. Channel congestion is also a classic culprit.
FIX First, log into the router and go to Wi-Fi Settings. Temporarily turn OFF "Smart Connect." This will show you three separate networks: YourNetwork_2.4G, YourNetwork_5G, and YourNetwork_6G. Connect your device specifically to the _5G or _6G network and test again. Next, check for active devices or tasks: in the Linksys interface, look under Device List and Parental Controls for any active heavy transfers or limits set.
IF THAT DIDN'T WORK The auto-selected Wi-Fi channel might be crowded. In the Wi-Fi Settings, for each band, change the Channel Selection from "Auto" to a specific channel. For 5GHz, try 36, 40, 44, or 48. For 2.4GHz, try 1, 6, or 11. Test after each change.
HEADS UP The speedtest server you use matters. For a valid test, use the same server (like one from your ISP) on both the wired and wireless tests. Also, wireless speed will always be less than wired due to overhead—that's normal physics.