I have an Elgato Eve Weather sensor on the balcony to check the weather with Eve iOS app. After I updated my iPhone to iOS 12 the Eve app could not find the sensor anymore. I even reseted the network setting in iOS (don't do that unless your are absolutely sure that there is no other way to fix your issue).
So I decided to contact Apple Support over Twitter. The person on Twitter mentioned firmware upgrade. I checked Elgato's page and looked for the PDF with instructions for my sensor if there are any mentions about firmware upgrade. Thanks to the PDF I found out that the sensor has a RESET button.
I reset the sensor and the iOS app found it again.
Speaking of Apple Support on Twitter, it is very responsive, I got the first answer after only a few minutes.