We definitely wanna see you stay connected. I have a few suggestions. First off, do you have any cases or covers on the phone? If so, would you mind taking the case/cover off to see if that helps out?
If this only happened after the update, it typically recommended to Master reset the phone and test it out with only the pre-installed apps.
I've used the same case/cover since I bought the phone months ago, the disconnection problem appeared only about a week ago. Also the signal seems fine, the problem arises only after leaving the wifi area.
I hoped there was a less invasive solution than master reset it.
I totally hear ya.
Those were just some steps we could use to help us figure out what's going on. If the only thing that's changed over the last week is a software update, a reset can help. If you've downloaded any apps recently, you could try removing them, restarting the phone and giving it another shot. It's a stretch, but sometimes 3rd party apps can do some pretty strange things and could be what's causing this.
Another thing you can test (if possible) is seeing if the same thing happens when you're leaving a Wi-Fi connection in another area. It's possible the location is the culprit. If it's happening in different areas and only after you're going from Wi-Fi to the mobile network, we'd be back to looking at continuing to troubleshoot the phone.