The LCD icon just indicates it’s connected to WiFi, it doesn’t indicate the signal strength. That icon would be either 4 bars (indicating it’s connected to WiFi), or 4 bars with a cross (indicating it has no WiFi connection).
The signal strength is a dynamic thing, it may not stay the same all the time. Does it persistently report Poor/Unusable? It may improve over time.
You can get an OpenSprinkler top-level logic board with a WiFi antenna plug, if you want.