On the MB there are two network cards, but in OpnSense only one is GigaEthernet.
Why?
The MB is a Gigabyte GA-C847N with 2 Realtek GbE
=== Update ===
The third additional card is also identified as 100Tx, but it is a GigaEthernet (DgLink DGE-528T)
=== New update ===
I have changed the add-on card and now this is 1000. The other one on a PC works correctly, so in OPNSens it's a drivers problem.
The one on the motherboard now works correctly at 1000 without me doing anything. Strange.