hi @msv_hofmann,
you need to connect ETH_2_RGMII_MDC and ETH_2_RGMII_MDIO to be able to access the registries of the switch. What you get from the mii command on Uboot are is most likely from the on-module Phy for the first ethernet. I also can find FEC1 with my Verdin iMX8MP on Yavia Board (which only has 1 Ethernet).
I am supporting @KDehren on this thread. Feel free to follow.
@matthias.tx can continue to support you if you need more help on the hardware side.