This solution worked for me. NB always check the beta Feedbck app for notices as one was posted about updated WatchOS before updating iOS. Here is the solution posted on a similar forum:
Mikis1Aug 14, 2019 12:50 PM(
in response to jkaye)
It can be easily solved in some basic steps without restoring anything neither watch or iDevice (Sorry for my simple English)
1. Download iOS 13 beta 5 from the internet (just type in google and you sure find something)
2. Choose your phone on the website and wait for download
3. Connect your phone and open iTunes
4. Once you are in your iDevice menu hit the update button BUT hold alt (In case of Windows CTRL) and then click it.
5. Choose downloaded file of iOS.
6. Wait for update to complete
7. Connect to your Apple watch and update them to newest software
And Voila! You're done and fully functional
Reason why is this still working is because Apple is still signing older betas and that's why you can still install it on your iPhone.
Have a nice day!