Why does my iPhone 7s keep losing Wi-Fi since I installed iOS12?

Before doing anything, have you erased network settings and verified WiFi is stable?

Next, erase all content and settings and retest on a new network.

Apple typically stops signing the previous release within a week of a new iOS, so you could try restoring back to iOS 11 to rule out a hardware failure that was a coincidence. At that point you can decide to see Apple service before upgrading to understand if you have any recourse from them in case a second upgrade also fails.

Many many iPhone 7 era devices work fine, so it’s almost certainly not a widespread bug. If you’re sure you want iOS 12 I would erase and restore the device and then connect to a Mac to look over the system logs if the failure continues.

Thanks, the reset of network settings did it. I'm annoyed at myself because I was so sure it was duet iOS12 that I didn't do theobvious.