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).
ESP8266 serial WIFI wireless transceiver module MT7681 3DBI gain antenna
ESP8266 serial WIFI wireless transceiver module MT7681 3DBI gain antenna
WeMos D1 mini PRO analog, WiFi D1 mini PRO integration of ESP8266 + 32Mb flash and USB-TTL CP2104
WeMos D1 mini PRO analog, WiFi D1 mini PRO integration of ESP8266 + 32Mb flash and USB-TTL CP2104