problems with wifi

I have several problems with wifi networks which I suppose is because I do not have some concepts. The times I install a Wi-Fi network with a router tplink or linksys, where I only had to configure the band, the name of ssid, if I used a dhcp and something else. Since I am using the mikrotik I saw that there are many options to configure.
For example there is a company where I have an internet connection of 50mbs and if I connect by wifi this connection drops to 20 mb.
I can not run a mikrotik configured as an ap in a room of 10 meters by 10 meters.
For it to work well I have to be about 3 meters away, if I move away the connection starts to fall.
Is there anything I can set to optimize this?.
The ap have a utp connected to the lan