Using the latest version of Teamviewer on a Windows 12 PC I am having a lot of issues sharing the screen on both my iPhone X and iPad Pro. I can connect just fine, but cannot broadcast the screen. It worked fine before last month's update to iOS 12. Now it works about 5% of the time on my iPad and about 20% of the time on my iPhone. I am unable to determine why it works sometimes and fails others.
I'm had the same issue. First time user, trying to connect from a Mac (Mohave) with TV 14 to an iPhone 7 with TV QuickSupport, iOS 12.1. On the iOS screen with "Screen Broadcast", the "TeamViewer" line was missing until I used the "Send ID" button, then copied/pasted the ID from the transmitted text file into the TV window on the Mac. On the earlier attempt after manually typing the ID into the TV screen on the Mac, the "connection" succeeded and i could "chat" with the iPhone, but when i touched "Screen Broadcast" on the iPhone, all it did was record a video of the iPhone screen to the iPhone itself. Repeating the connection sequence, this time touching "Send ID" on the iPhone apparently did something to the connection that enabled more than just chat (i.e. screen sharing).
Press on the 3 dots up top and select advanced, here you must turn off the UDP slider.
After doing this, the teamviewer app showed up in the Screen recording icon and the screen was being shared.
* I could not remotely control the device, all i could do was share the screen.
After doing this I could share the screen constantly.