I saw that
@KakatkarAkshay already wrote an info about this case in his main post on page 1:
Note: If WiFi/Network does not work for you then
you must Flash OxygenOS 12 and then Flash this ROM.
So finally i re-flashed the newest OxygenOS Stock ROM (AC2003_11_F.18) on both slots to be on the latest official firmware build. I made a backup first (Take SwiftBackup if you have rooted your phone or just the built-in Google Backup solution).
After this I clean flashed the latest PixelExperience and now everything is working fine again.