Be sure you have a good cable between the modem and router.

Placement of the router as to location, height, direction it faces, direction of antennas if any affects distance and coverage.

WIFI has two Bands - 2.4 GHz and 5 GHz. 
The 2.4 GHz is slower but works at father distances. 
The 5 GHz is faster but not does work as well at distance. 
The SSIDs should be different for best performance and enables you to put specific devices on a specific band. For example, you may want to put printers on the slower band and your PC on the faster band. 
WIFI devices have different standards such as 802.11A, 802.11B, 802.11G, 802.11N, and 802.11AC. “A” and “C” are the oldest and very rare if any devices uses it today.
“G” is the older and slowest of devices in use today. Next was “N’ which is faster. The current standard is “AC”. Routers will support the older versions such as a “N” router will support both “G” and “N” standards and an “AC” router will support all previous standards. And “AC” routers also have speed ranges. 
Also on some routers, WIFI will slow down to the slowest connected device so if you have an old “G” device, it may be slowing your WIFI down to “G” speeds. Power off all of your WIFI devices and test with only your test device connected.
Then of course the device you are testing the WIFI speed may be an issue as the WIFI driver in the device may be the limiting factor.

Other devices may be interfering with the radio signal. Things like cordless phones, wireless security cameras, anything wireless device that uses the 2.4 or 5 bands. 

Other WIFIs may be flooding the channel your WIFI is using. Each location will have its own best channel and that can vary from time to time. I have a iMac and it has tools (although somewhat hidden) that allow me to see the other nearby SSIDs and the bands/channels they are using. There are WIFI scanners that you can install for Windows machines. I have my router channels set to Auto which works okay for me.