If you have deactivated your W-Lan connection via the status bar of your Samsung Galaxy S9, you may notice after a short time that the W-Lan connection is active again. The reason for this can be different, but one possibility is definitely an activated Wi-Fi calling.
If this option is activated, calls are handled via the W-Lan network at home. Therefore, if you think your W-Lan will always turn on automatically, try the following:
Test deactivating W-Lan calls
1. Open the phone app on the smartphone
2. Tap the three-point icon at the top right - a pop-up menu appears
3. Select in this "Settings"
4. Scroll down and deactivate the slider for "WLAN calls" This will give you the necessary settings on the Samsung Galaxy S9.
From now on, your Samsung Galaxy S9 should no longer automatically establish a wireless connection if you have previously disabled it. Since this is most likely a bug in the firmware of the S9, this will most likely be fixed soon.
Until then, however, the above procedure will help you.