First ensure that the sensor status light is orange (this signals that the config mode is active).
If you don't see a "koubachi_config" network in your wireless networks list, read the following article.
If the "koubachi_config" network shows up, select the entry and wait up to 30 seconds for the connection to be established. If it should not work, switch your Wi-Fi off, "restart" the configuration on the plant sensor, and switch the Wi-Fi back on again.
If the previous did not work for you, try the configuration with another device if available.
Expert note: you can verify the connection by pinging the device with "ping 172.29.0.1"