Hi @kult!
A way to control GPIOs is by using gpio-hog in a device tree overlay and setting the pin configuration in the node. Please refer to this documentation to see how to use gpio-hog and an example of its usage.
It is important to remember that the GPIO must be “free” to use it in gpio-hog, i.e., it cannot be called by any other node, so be sure the GPIO you want to use isn’t being used, and if it is, you can disable the node that is using the GPIO by setting the property status = "disabled";.
If you want to use a GPIO that is set as another function, please refer to the pin multiplexing article to learn how to change a pin functionality.
Regarding the second ethernet, which module (SoM) and carrier are you using?
Best regards.