ESP8266 Wemos D1 mini PRO board has an external antenna connector, besides the internal antena. The external antenna is not enabled. To do this, you must move the zero value resistor R16.
Notice the position were it should be the resistor for enabling external antenna connector:
Details before and after soldering
Follow the next article for a test program to read the wifi signal strength RSSI (Received Signal Strength Indicator).
The external antenna is from here
ESP8266 serial WIFI wireless transceiver module MT7681 3DBI gain antenna
ESP8266 D1 mini PRO extenal antenna