how do I use H11L1 correctly to receive MIDI?
You cannot use both serial monitor and the serial input pin. They share the same serial port. 

Even not used, TX from MIDI conflicts with TX from PC at the microcontroller RX input which keeps the input to logic "1" . Reading as digital input doesn't change anything. 

Use a software UART for MIDI or find some other way to test your program without using  the serial monitor. 

The software UART can help you fix the design flaw noticed also by @Transistor that the output is inverted by inverting the input in software.