Hi Timon,
two common errors (among others) could cause the GPS not to be recognised:
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The TX/RX lines must be cross-connected. So TX of the GPS to RX of the FC.
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Unfortunately, the Mateksys naming of the serial interfaces does not match the serial numbering of Ardupilot: Serial 3 of Ardupilot corresponds to the TX2/RX2 pins of the board (Flight Controller H743-WING V2/V3 – Matek Systems)