@dc42 Thank you, that pointed me into the right direction.
I tried to use a different circuit for the Heatbed as the Main Vin of the board has a 6A Breaker.
The Heatbed is now switched externally by a separate Mosfet, this works perfectly, also directs some of the High Current Stuff away from the Duet.

@dc42 I am using the Internal Mosfet, +24V are connected to VIN of the Heatbed Terminal, V+ of the Heatbed is connected to the other Side of the Teminal.
The Heatbed then has its V- connected directly to Ground.

But the thing I am really concerned with, is the missing Gate Signal.

Hello there,

Machine Setup is the following:
Vin: 24V
Heatbed Current Max: 11A (Switched on the High side)

Heater Config:
M305 P0 T100000 B4138 C0 R4700 ; Set thermistor + ADC parameters for heater 0
M143 H0 S120 ; Set temperature limit for heater 0 to 120C
M305 P1 T100000 B4725 C7.060000e-8 R4700 ; Set thermistor + ADC parameters for heater 1
M143 H1 S280 ; Set temperature limit for heater 1 to 280C

FW:
Electronic: Duet WiFi 1.02 or later
Version: 1.21 (2018-03-21)
Wifi Server: 1.21RC4(08b3)
Web Interface: 1.21-RC4

I just finished installing my Duet 2 Wifi.
Problem is that the Heatbed is not Heating up, first i measured the Voltage Across the Terminals, that turned out to be a constant 24V (also when bedheating was on 55°C).
Then i measured the Gate Voltage on R20 as well as the Gate of the Mosfet (Infineon IPD036N04L G).
Both Turned out to be 0V.
The Heatbed Led is constantly On during all tests (makes sense when the Mosfet isn´t switching).
Seems to me that the PWM-Signal of the Micro isn´t reaching the Mosfet...

Any Ideas on how to fix this?

Sincerely,
Slaycrit