How to turn the liquid detection setting back on

There is no function in Settings that can activate Liquid Detection. If you do not see Liquid Detection when you should, contact an authorized Apple repairer.

See this Support Article:

If you see a liquid-detection alert on your iPhone - Apple Support

I doubt if it is a "One time solution". By choosing Emergency Override, you choose to accept that the phone may be damaged when charging. If you haven't charged the phone, make sure it's dry before plugging in a charger again.

From what I can find of information, you will be prompted for it every time if water is detected. But as I said, there is no function that can activate/reactivate the function.

Start by drying your iPhone and make sure it is completely dry before plugging in a charger, also see what happens. Hopefully you'll never see Liquid Detected on your screen again.