It has been a long time, but I finally found a solution!
When my connection is in Hybrid Mode (LTE+DSL together), 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 this line to the OpenVPN directives in Eddie.
OpenVPN over UDP --> "mssfix 1356"
// Hybrid MTU 1384 - (UDP Header 8 Byte + IPv4 Header 20 Byte) = 1356
OpenVPN over TCP --> "mssfix 1344"
// Hybrid MTU 1384 - (TCP Header 20 Byte + IPv4 Header 20 Byte) = 1344
I know that the TCP header can be up to 60 byte large, but for me 1344 works just fine.
I can now use UDP protocol without any issues and even TCP now is almost as fast as UDP.
In case there are any other DSL Hybrid users out there having trouble with OpenVPN speeds.