How to establish transformer less connection between Openwrt and linux

I am using a carambola 2 hardware router with OpenWRT. here, I tried to establish a transformer-less connection with the stm32 chipset.

when auto-negotiation is ON, the connection is established but no IP is available.

I tried setting auto-negotiation to OFF, and the speed to 100Mb/s, but the speed of eth0 automatically goes to 10Mb/s.

I would like to know what I could do to establish the connection between the two devices.

Thank you in advance