It has been a long time, but I finally found a solution!
When my internet connection is in Hybrid Mode (LTE + DSL bonded), the MTU value is 1384.
In order to use AirVPN with full speed (no mather if using UDP or TCP protocoI), I just had to apply the mssfix parameter to the OpenVPN directives in Eddie.
The following solution might also work for every any other Fiber / Cable / DSL / LTE / ... internet connection, on which the MTU value is below 1500. At my university for example, I have to do the same but with different values.
Check your MTU value without being connected the the VPN. https://www.speedguide.net/analyzer.php
This got me a value of 1384.
OpenVPN over UDP --> "mssfix 1356"
// MTU 1384 - (UDP Header 8 Byte + IPv4 Header 20 Byte) = 1356
OpenVPN over TCP --> "mssfix 1344"
// MTU 1384 - (TCP Header 20 Byte + IPv4 Header 20 Byte) = 1344
I know that the TCP header can be up to 60 byte, but for me 20 worked just fine.
I can now use UDP protocol without any issues and TCP is almost as fast as UDP.