Wireless speeds capping at 20 Mbps on preferred plan?

Hi,

We have a Arris SB6121 modem and a TP-Link TL-WR841N router with our preferred internet plan. We have no complaints about speeds with an Ethernet cable, but wireless speeds have a bit more disappointing, as they've never gone past 20 Mbps which is far from the 50 of the plan. We initially thought it was due to the location of some devices in the apartment, but even devices right next to the router are experiencing those speeds. Is there a possible fix for this?

Cheers.